Senior man is a position held in some collegiate universities by the head of a college's junior common room (JCR, the undergraduate student body).
The senior man acts as the chief representative of the student body, as a figurehead for the JCR and as an elected official.
[1] The senior man is also responsible for leading the JCR's executive council.
[2] This reflects the history of these colleges, which were founded as single sex male-only institutions.
Prior to this change, and the same with Hatfield College, female students had held the role of Senior Man.