A guru (Sanskrit: गुरु) (Gurmukhi: ਗੁਰੂ) is one who is regarded as having great knowledge, wisdom and authority in a certain area, and who uses it to guide others (teacher). Other forms of manifestation of this principle also include parents, school teachers, non-human objects (books) and even one’s own intellectual discipline.
In the religious sense the term is commonly used in Hinduism, as well as in other Indian religions and new religious movements. Finding a true guru is often held to be a prerequisite for attaining self-realization. In contemporary India, the word “guru” is widely used with the general meaning of “teacher”. In Western usage, the meaning of guru has been extended to cover anyone who acquires followers, though not necessarily in an established school of philosophy or religion. In a further Western extension, guru is used, or even misused from the original religious meaning, to refer to a person who has authority because of his or her perceived secular knowledge or skills, such as in business.
Definition courtesy Wikipedia.