Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Senior \Sen"ior\, n.
   1. A person who is older than another; one more advanced in
      life.

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

   3. An aged person; an older. --Dryden.

            Each village senior paused to scan, And speak the
            lovely caravan.                       --Emerson.

   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.

Source : WordNet®

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}]