WordNet (r) 3.0 (2006):

leave of absence
    n 1: the period of time during which you are absent from work or
         duty; "a ten day's leave to visit his mother" [syn:
         leave, leave of absence]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "leave of absence":
   AWOL, French leave, abscondence, absence, absence without leave,
   absentation, absenteeism, absenting, cut, day off, default,
   departure, disappearance, escape, excused absence, fleeing,
   furlough, holiday, hooky, leave, leaving, liberty, nonappearance,
   nonattendance, paid holiday, paid vacation, running away,
   sabbatical, sabbatical leave, sabbatical year, shore leave,
   sick leave, time off, truancy, truantism, unexcused absence,
   vacation, weekend