은사가 · 은사를
one's (respected) former teacher
Example
졸업하고 20년 만에 고등학교 은사님을 찾아뵈었어요.
Twenty years after graduating, I visited my old high school teacher.
Word parts
은 grace+사 teacher
Literally 'teacher of grace', a teacher you owe much to.
Good to know
Usually said with -님: 은사님; 찾아뵙다 is the humble verb for visiting them.