One of the primary definitions of a guru is a teacher with knowledge and expertise. 

A guru has studied and learned everything possible about one topic. They have dedicated their lives to having knowledge that few others obtain. Then they willingly share what they have learned with others.

The trouble comes when they think their knowledge transfers to other areas. 

This could be a sports star talking about politics, an actor speaking about writing, or even a preacher talking about things not found in the Bible.

Be careful whose voice you listen to for advice, teachers abound, but only a few are true gurus.