Guru
गुरु (guru)
Teacher - literally one who dispels darkness, naming a relationship of transmission, not just instruction.
Guru is often glossed simply as teacher, which captures the role but loses the etymology. One common reading breaks the word into gu (darkness) and ru (that which dispels), naming not an instructor of facts but one who removes an obscuration the student could not remove alone.
This is a heavier claim than the English word teacher usually carries, and it explains why the guru relationship in many traditions is treated with a formality and duration that ordinary instruction is not. It is not assumed that darkness dispels in a single lecture.
The modern casual use of guru, applied to any expert in any field, has thinned the word considerably. Read in its original register, it names something closer to an accompanied transformation than a transfer of information.