The Collaborative International Dictionary of English v.0.48:

Mandatory \Man"da*to*ry\, n.
   Same as Mandatary.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mandatory \Man"da*to*ry\, a. [L. mandatorius.]
   1. Containing a command; preceptive; directory.
      [1913 Webster]

   2. Obligatory; compulsory; required by authority.
      [PJC]

   3. (Law) Not optional; not able to be modified or
      disregarded; as, seven mandatory clauses in the contract.
      [PJC]

WordNet (r) 3.0 (2006):

mandatory
    adj 1: required by rule; "in most schools physical education is
           compulsory"; "attendance is mandatory"; "required
           reading" [syn: compulsory, mandatory, required]
    n 1: the recipient of a mandate [syn: mandatary, mandatory]
    2: a territory surrendered by Turkey or Germany after World War
       I and put under the tutelage of some other European power
       until they are able to stand by themselves [syn: mandate,
       mandatory]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "mandatory":
   absolute, authoritative, binding, canonical, choiceless, commanded,
   commanding, compelling, compulsatory, compulsory, conclusive,
   de rigueur, decisive, decretory, demanded, dictated, didactic,
   entailed, essential, exigent, final, forced, formulary,
   hard and fast, hard-and-fast, imperative, imperious, importunate,
   imposed, indispensable, inevitable, instructive, involuntary,
   irrevocable, mandated, must, necessary, necessitous, needed,
   needful, obligatory, official, peremptory, preceptive, prescribed,
   prescript, prescriptive, regulation, required, requisite, rubric,
   standard, statutory, ultimate, urgent, without appeal,
   without choice