[1] : 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
see also:
sir 

[2] : 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
see also:
senior sophister 

[3] : WordNet (r) 2.0

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: seniora, 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
     n 1: an undergraduate student during the year preceding
          graduation
     2: a person who is older than you are syn: elder
see also:
junior seniora fourth-year aged elderly older 
elder 

[4] : 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

[5] : 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. 

Senior >> S Words
Senior, definition of term: Senior
senior_pag1.html

Senior Citizen >> S Words
Senior Citizen, definition of term: Senior Citizen
senior+citizen_pag1.html