Noah Webster's New International Dictionary of the English Language

1. (a.) More advanced than another in age; prior in age; elder; hence, more advanced in dignity, rank, or office; superior; as, senior member; senior counsel.

2. (a.) Belonging to the final year of the regular course in American colleges, or in professional schools.

3. (n.) A person who is older than another; one more advanced in life.

4. (n.) One older in office, or whose entrance upon office was anterior to that of another; one prior in grade.

5. (n.) An aged person; an older.

6. (n.) One in the fourth or final year of his collegiate course at an American college; -- originally called senior sophister; also, one in the last year of the course at a professional schools or at a seminary.