In Sanskrit, Guru word is made up of two root words Gu and Ru. Gu means darkness, and Ru means remover. Thus, guru stands for the teacher who is the remover of darkness and is the harbinger of enlightenment.
The auspicious mantra recited an chanted on teacher’s day and Guru Purnima is "Guru Brahma, Guru Vishnu, Guru Devo Maheshwara; Guru Sakshat Param Brahma, Tasmai Shri Guravay Namah".
Guru Brahma - Guru is Brahma, who is the Lord of Creation, also called as Generator, Guru Vishnu means Guru is Vishnu (Vishnu is the Lord who is called organizer), Guru Devo Maheshwarah means Guru is the Maheshwara (Shiva or the destroyer), Guru Sakshat Parabrahma means Parbrahma viz. the supreme god or almighty.
Since Guru leads to a path of light, Guru is that Para Brahma. Tasmai Shree Guruve Namah means we bow to that Guru the guru referred to earlier.
Thus, as a realized soul, Guru is the embodiment of Para Brahma, the ultimate Godhead.