Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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