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]

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]

WordNet (r) 3.0 (2006):

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]