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]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "senior":
   A per se, absolute, ace, ancient, antecedent, anterior,
   anticipatory, ascendant, authoritarian, authoritative, authorized,
   autocratic, better, boss, brass hat, cadet, champion, chief,
   ci-devant, clothed with authority, commander, commanding,
   competent, consequential, considerable, controlling, dean,
   dominant, doyen, doyenne, duly constituted, earlier, early, elder,
   eldest, eminent, empowered, ex officio, father, first, first-born,
   firstling, fore, foregoing, former, freshman, fugleman, genius,
   golden-ager, governing, great, head, hegemonic, hegemonistic,
   high priest, higher-up, imperative, important, important person,
   influential, junior, kingfish, kingpin, laureate, leader, leading,
   major, master, midshipman, mighty, momentous, monocratic,
   nonpareil, official, old-timer, older, oldest, paragon, personage,
   plebe, potent, powerful, preceding, precurrent, preeminent,
   preexistent, prestigious, previous, prime, primogenitary,
   principal, prior, prodigy, prominent, puissant, ranking, ruler,
   ruling, senior citizen, sire, soph, sophomore, star, substantial,
   superior, superman, superstar, supreme, the greatest, the most,
   top dog, totalitarian, undergrad, undergraduate, upperclassman,
   virtuoso, weighty



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SENIOR. The elder. This addition is sometimes made to a man's name, when two
persons bear the same, in order to distinguish them. In practice when
nothing is mentioned, the senior is intended. 3 Miss. R. 59. See Junior.