THE term Dev is derived from the Sanskrit root Div meaning "Light." The words Guru Dev are therefore used to denote the astral or luminous form of the Master as he appears in astral regions, after leaving the physical plane to guide the spirit as it transcends the physical body.
    Theosophical literature refers to the luster of Master Souls in subtle and causal planes extending for miles and miles. Tulsi Sahib likewise tells us that the nails of the Guru's feet shine like Mani (a crest jewel in the head of a toad that shines forth like a flashlight in darkness). This astral light bestows vision to the third eye.

        Blinding Light flashes forth from the nails of the Master's Feet
         and illumines the very soul of the devotee.

Maulana Rumi speaks of It:

        As the Light of the Master dawns in the soul, one gets to know
        the secrets of both worlds.

    A real Guru is the true manifestation of God. He is, in fact, Satguru or Master of Truth, and manifests in the world the Light of Truth.

        O Nanak! Guru is the Satguru;
        I would like to touch the Feet of the Satguru.

    The term Guru Dev, therefore, signifies the self-luminous form of the Master, which is free from and far above his physical body, and which the spirit actually perceives with its inner subtle Light. When the spirit comes face to face with the astral Master, all doubts vanish and its labors get the crowning reward - the summum bonum of life.

        Guru Dev gives vision to the eyes. All doubts become airy nothings
          and love's labor receives its crowning glory.

Jesus Christ in the Gospel speaks thus:

        If therefore thine eye be single, thy whole body shall be full of light.

                                                                                                                                         MATTHEW 6:22

Guru Arjan tells us that the Blessed Form of the Master manifests itself in the forehead of a devotee.

        The Blessed Form of the Master is in my forehead.
        Whenever I peep within I see him there.

A Muslim Divine also speaks like this:

        The image of the Beloved is in the mirror of my very heart. I see him
         with just a little turn of the head downward.

It is this astral or self-luminous form of the Master that leads a spirit Godward. through various planes intervening between the physical plane and Sat Lok or the Region of Truth. There is no distinction between Guru and Guru Dev and Satguru and God.
    It is one and the same current of divine compassion that assumes different names at different regions.
    Following the law of similitude, the Divine Current as It materializes on the physical plane for the benefit of the aspirants is called Guru or Master, who by word of mouth imparts the spiritual instructions as any other teacher would do.
    As the spirit of an aspirant leaves the body and is ready for the journey in the astral or subtle planes, that very Divine Current assumes a subtle form for the soul's benefit and guidance.
    This subtle form disengaged from the bodily form of the Guru is termed Guru Dev. It is self-luminous and lustrous with Light extending over miles and miles. Satguru or the Master of Truth is the Power of Truth or God that works through both Guru and Guru Dev. With roots firmly embedded in Sat or Truth, he directly derives his inspiration from the eternal and unchangeable permanence, Sat, hence is known as Satguru.
    Thus we see how the Sat Dhara or the current from Sat flows down, creating regions upon regions far below, ending with the physical plane.
    It is this very current that also helps in taking the jivas back home, and is differently known as Guru, Guru Dev and Satguru, at different places, until the very roots of Sat are reached, and here the spirit cries out in wonder Wah-i-Guru, which means, "What a glory is thine, O Guru!"
    Indescribable and beyond comprehension! So we have in Gurbani:

        Guru Dev is Satguru, Par Brahm, and Parmeshwar. O Nanak! a Salutation
          to Guru Dev is Salutation to Hari or God.

This very Sat or Truth is at one with the Guru as he works on the physical plane. Hence, it is said:

        Blessed is the physical form of the Master, filled as it is to the brim
         with full power from above.

        The Greatness of the Master is indescribable and beyond all comprehension,
         for he is Par Brahm, Parmeshwar, beyond perception and knowledge.

        Guru Dev can neither be known nor measured. By following the
         instructions one can penetrate far into the mysteries of the past,
         the present, and the future. It is through his sheer Grace that one
         begins to know something of the unknown and the unknowable.

In the physical, world he works and acts like a Guru or Master; but when a jiva after some spiritual practice and discipline is ready to leave the Pind or body and is about to enter And (the subtle plane), the Guru comes to his aid in subtle shining form as his Guru Dev. Here he works as a connecting link between Guru and Satguru for he takes over the charge of a spirit from the Guru in the body, and leads her across to Satguru and Sat Purush.
    Guru Dev meets and greets the spirit as it crosses the border lying between the physical and subtle regions by passing through the stars, the sun and the moon, spoken  of in the Vedas as Devian and Pitrian Margs (Paths). This astral form is exactly like the physical form of the Master, but much more beautiful, luminous, and magnetic.
    Maulana Rumi tells us:

        Should you desire to see this refulgent Light, turn homeward
         like Ibrahim. Pass through the big star and the sky and the blue
         beyond. Steadily walk over the sun and the moon, and then you
         will find yourself in the heavenly Presence.

Guru Nanak refers to this illuminated path, thus:

        The Luminous Form of the Master is wondrously enrapturing and
         enchanting. Only a Perfect Master can manifest this to a spirit.

This lustrous form of the Master always accompanies a spirit in the various planes, ending with Sach Khand or the Home of Truth. When his luminous form descends to the focus of the eyes, a devotee has nothing more to strive for. Herein lies the devotion of the devotee. Half his success has been achieved, and hereafter the Master's astral form takes over the charge of the spirit with full responsibility for leading it to the final goal. Even the Saints also adore this form and derive   ecstatic delight from it.

        The Blessed Feet of the Beloved Guru Dev are worshiped by the Saints,
         the beloved of God.

Khwaja Moeen-ud-Din Chisti also talks of the Luminous Form of the Master:

        O Master! The sun cannot stand the resplendence
          of Thy Face.
        The moon also has covered herself with clouds
          to escape Thy Dazzling Light.
        The sun having borrowed his brilliance from the dust of Thy Feet
          has pitched his golden pavilion on the blue sky. If a single ray of
          Thy Face were to shoot forth into the sky, the sun would in shame
          go behind a veil. In the very Person of the Nabi (Prophet), the Light
          of God has taken up a material form just as the light of the sun
          does in the body of the moon.

Maulana Rumi also refers to the lustrous form of his Master thus:

        What dost thou know of the King of Kings that keeps me company?
        Cast thou a glance within me and be not deceived by my outer appearance.

St. John has similarly described in the Bible his experience with the Luminous Form within:

        I was in the Spirit ... and heard behind me a great voice, as of a trumpet,...
        And I turned to see the voice that spake with me. And being turned,
          I saw ... one like unto the Son of man, clothed with a garment down
          to the foot, and girt about the paps with a golden girdle.
        His head and hairs were white like wool, as white as snow; and his eyes
           were as a flame of fire;
        And his feet were like unto fine brass, as if they burned in a furnace;
           and his voice as the sound of many waters....
        And his countenance was as the sun shineth in his strength.

                                                                                                                       REVELATION 1:10, 12-16

        After this I looked, and, behold, a door was opened in heaven:
          and the first voice which I heard was as it were of a trumpet talking
          with me; which said, Come up hither, and I will show thee things
          which must be hereafter.

                                                                                                           REVELATION 4:1

In the Sar Bachan of Swami Ji Maharaj, we have a similar reference:

        Wondrous strange was the Form; No words could this Glory write.

Hafiz, a mystical poet of great renown, tells us:

        Like the newborn moon the pure of sight may see;
      His Glory would not manifest to every eye.

The astral form of the Master Soul is unchangeable and permanent. It is the form that guides aspirants to their goal.

        Guru Dev is in the beginning of creation. He is in the beginning of each age
         and continues from age to age. Through Guru Dev alone can one reach Hari.

Guru Arjan speaks of Guru Dev:

        Salutations to Guru Eternal (Har Rai)
        Salutations to Guru of the Age (Sat Purush or Ram Rai);
        Salutations to Satguru (the manifestation of Sat Purush); and
        Salutations to Guru Dev (the Refulgent and Self-Luminous Form
          of the Guru, the connecting link between Guru and Satguru,
          responsible for guiding and leading a spirit through various planes).

Guru Dev is the greatest and highest manifestation of Sat Purush. He is the controlling power of God and can grant salvation. By devotion to him one gets all comfort.
    Nothing but Sat is Guru Dev; false is all worship apart from his.
    Guru Arjan in his memorable words sings of the greatness of Guru Dev thus:

        Gurudeva is father; Gurudeva is mother;
           and he is the Master and God Himself;
        Gurudeva is a real friend;
           dispeller of dark ignorance and breaker of all ties;
        Gurudeva is the bestower of Naam, an incantation of which
           chases away all evils;
        Gurudeva is the embodiment of peace, truth and intelligence,
          and truly a philosopher's  stone;
        Gurudeva is a place of pilgrimage, fountain of the Elixir of Life,
          and light of reason;
        Gurudeva ordains everything, destroys sins and purifies the sinners;
        Gurudeva is ever eternal from the creation and beginning of each age,
          and his Word has a saving Grace;
        Gurudeva is the greatest gift of God, which, if granted, saves
          the worst sinners;
        Gurudeva is Satguru, Par Brahm, and Parmeshwar.
        Salutations to him, O Nanak.

Gurbani tells of the immense benefit that one enjoys on meeting Guru Dev:
    The five deadly sins-desire, anger, greed, infatuation, and egoism - just vanish.
    Myriads of Karmic impressions of countless ages are destroyed. He pulls a jiva from body and mind consciousness and ushers it into cosmic awareness, where it no longer experiences the fires of the world in which the entire humanity is trapped.
    Even in the world all the soul's desires are fulfilled. His course is soft and smooth thereafter, and receive acclamations from every side.
    In this Kali Yuga (Iron Age) of iniquity and darkness Guru Dev serves as a beacon light on the storm sea of life, and safely pilots sinners to the Heaven of peace and bliss. A saved soul in its turn pulls along all who are near and dear to it.
    The manifestation of Guru Dev depends purely on the grace of God and one's special merits in one's progress on the spiritual path.