恩师 (ēnshī): one's revered teacher

A warm and reverent term - much more loaded than 老师 (lǎoshī, the everyday word for 'teacher'). 恩师 is reserved for a specific teacher whose influence shaped you: a thesis advisor, a martial-arts master, a mentor who took you in. Often paired with 终生 (lifelong).

culture

Sits inside a Confucian-era set of debt-words built on 恩 (en, 'kindness/favor that demands repayment'): 恩师 (debt-teacher), 恩人 (benefactor), 报恩 (repay a debt of gratitude). The relationship implies lifelong obligation, not just past instruction.