From WordNet (r) 3.0 (2006) [wn]:
senior
adj 1: older; higher in rank; longer in length of tenure or
service; "senior officer" [ant: junior]
2: used of the fourth and final year in United States high
school or college; "the senior prom" [syn: {senior(a)},
fourth-year]
3: advanced in years; (`aged' is pronounced as two syllables);
"aged members of the society"; "elderly residents could
remember the construction of the first skyscraper"; "senior
citizen" [syn: aged, elderly, older, senior]
n 1: an undergraduate student during the year preceding
graduation
2: a person who is older than you are [syn: elder, senior]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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 [gcide]:
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.
[1913 Webster]
3. An aged person; an older. --Dryden.
[1913 Webster]
Each village senior paused to scan,
And speak the lovely caravan. --Emerson.
[1913 Webster]
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]