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.
      [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]