gu·ru 
(gr, g-r)
(gr, g-r)n. pl. gu·rus
1. In Hinduism and Tibetan Buddhism, a personal spiritual teacher.
2.
a. Any of the ten successive spiritual teachers who developed Sikhism between the 16th and 18th centuries.
b. Guru See Guru Granth Sahib.
3.
a. A trusted counselor and adviser; a mentor.
b. A popular or influential leader or advocate, as of a movement or idea: "In a culture that worships slimness, he was the Guru of Lean" (Erica Abeel).
[Hindi guru, from Sanskrit guruḥ, from guru-, heavy; see gwerə-1 in the Appendix of Indo-European roots.]