[1] : The Collaborative International Dictionary of English v.0.48

obligatory \ob"ligatory\, a. l. obligatorius: cf. f.
   obligatoire.
   binding in law or conscience; imposing duty or obligation;
   requiring performance or forbearance of some act; -- often
   followed by on or upon; as, obedience is obligatory on a
   soldier.
   1913 webster

         as long as the law is obligatory, so long our obedience
         is due.                                  --jer. taylor.
   1913 webster

[2] : WordNet (r) 2.0

obligatory
     adj 1: morally or legally constraining or binding; "attendance is
            obligatory"; "an obligatory contribution" ant: optional
     2: required by obligation or compulsion or convention; "he made
        all the obligatory apologies"
see also:
optional 

[3] : Moby Thesaurus II by Grady Ward, 1.0

38 moby thesaurus words for "obligatory":
   absolute, binding, choiceless, compulsory, conclusive, de rigueur,
   decisive, decretory, demanded, dictated, entailed, essential,
   exigent, final, hard-and-fast, imperative, imperious, importunate,
   imposed, incumbent, indispensable, inevitable, involuntary,
   irrevocable, mandated, mandatory, must, necessary, necessitous,
   peremptory, prescript, prescriptive, required, requisite, ultimate,
   urgent, without appeal, without choice

Obligatory >> O Words
Obligatory, definition of term: Obligatory
obligatory_pag1.html