Definition - What does Adi Guru mean?
Adi guru is a Sanskrit term referring to the first guru, or spiritual teacher. The term is sometimes used to refer to “primordial masters,” or founders of philosophies and faiths like Islam, Hinduism and Greek philosophy.
In yogic philosophy, Lord Shiva is recognized as the Adi Yogi (first yoga practitioner) and the Adi Guru.
Yogapedia explains Adi Guru
As part of the divine Trimurti, Shiva takes on many forms in Hindu mythology and is regarded as the source for India's yogic traditions. As one related mythological story goes, Shiva reached enlightenment and became ecstatic. He was so ecstatic that he could not withhold himself from periods of passionate dance. These periods of dancing were then followed by blissful states of stillness. He continued this pattern of dance and meditative rest for years. During his expressions of ecstasy, onlookers came and went, but seven men devoted themselves to learning what wisdom Shiva had learned. Eventually, Shiva transformed into the Adi Guru in order to impart this yogic knowledge to his devotees.