The Collaborative International Dictionary of English v.0.48:

Compulsory \Com*pul"so*ry\, a. [LL. compulsorius.]
   1. Having the power of compulsion; constraining.
      [1913 Webster]

   2. Obligatory; enjoined by authority; necessary; due to
      compulsion.
      [1913 Webster]

            This contribution threatening to fall infinitely
            short of their hopes, they soon made it compulsory.
                                                  --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

compulsory
    adj 1: required by rule; "in most schools physical education is
           compulsory"; "attendance is mandatory"; "required
           reading" [syn: compulsory, mandatory, required]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "compulsory":
   absolute, binding, choiceless, coactive, compelling, compulsatory,
   compulsive, conclusive, constraining, decisive, decretory,
   dictated, driving, entailed, exigent, final, hard-and-fast,
   imperative, imperious, importunate, imposed, inevitable,
   involuntary, irresistible, irrevocable, mandated, mandatory, must,
   necessary, necessitous, obligatory, peremptory, prescript,
   prescriptive, pressing, required, restraining, ultimate, urgent,
   without appeal, without choice