Definition of “senior” - Learner’s Dictionary

senior

noun uk us /ˈsiːniər/
be 20/30, etc years sb's senior

to be 20, 30, etc years older than someone:

She married a man 20 years her senior.

[ C ] US a student in their last year of study at an American college or high school (= school for students aged 15 to 18)

(Definition of “senior noun” from the Cambridge Learner's Dictionary © Cambridge University Press)