From The Collaborative International Dictionary of English v.0.48:
Senior \Sen"ior\, a. [L. senior, compar. of senex, gen. senis,
old. See Sir.]
1. More advanced than another in age; prior in age; elder;
hence, more advanced in dignity, rank, or office;
superior; as, senior member; senior counsel.
[1913 Webster]
2. Belonging to the final year of the regular course in
American colleges, or in professional schools.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Senior \Sen"ior\, n.
1. A person who is older than another; one more advanced in
life.
[1913 Webster]
2. One older in office, or whose entrance upon office was
anterior to that of another; one prior in grade.
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3. An aged person; an older. --Dryden.
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Each village senior paused to scan,
And speak the lovely caravan. --Emerson.
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4. 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.
[1913 Webster]