Shiv explains himself as Guru(गुरू) in 'Shri Guru Gita'

यो गुरु स शिवः प्रोक्तो, यः शिवः स गुरुस्मृतः |
विकल्पं यस्तु कुर्वीत स नरो गुरुतल्पगः ||

He who is the Guru is Shiva Himself, so declare the scriptures, and the fact that Shiva is the Guru, is reminded to us in all the Smritis. He, who makes any distinction between the two, is guilty of the crime of uniting with his own Guru’s wife.

गुरू स्वयं शिव समान हैं, एवं शिव स्वयं गुरू हैं, गुरू एवं शिव अंतर स्थापित करना महा अपराध है |