The Collaborative International Dictionary of English v.0.48:

Seniority \Sen*ior"i*ty\, n.
   The quality or state of being senior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seniority
    n 1: higher rank than that of others especially by reason of
         longer service [syn: seniority, senior status, higher
         status, higher rank]
    2: the property of being long-lived [syn: longevity,
       seniority]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "seniority":
   aboriginality, accomplishment, age, ancien regime, ancientness,
   antiquity, ascendancy, atavism, authority, cobwebs of antiquity,
   consequence, deanship, dust of ages, eld, elderliness, eldership,
   eminence, excellence, favor, great age, greatness, hoary age,
   hoary eld, importance, incomparability, influence, influentialness,
   inimitability, inveteracy, lead, majority, moment, old age,
   old order, old style, oldness, one-upmanship, precedence,
   predominance, predomination, preeminence, preponderance,
   prepotence, prepotency, prerogative, pressure, prestige,
   primitiveness, primogeniture, primordialism, primordiality,
   priority, privilege, prominence, rank, right-of-way, senility,
   skill, stature, success, superiority, transcendence, transcendency,
   venerableness, virtuosity, weight, weightiness