Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Obligatory; enjoined by authority; necessary; due to
      compulsion.

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

Source : WordNet®

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