Source : Webster's Revised Unabridged Dictionary (1913)

Obligatory \Ob"li*ga*to*ry\, 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.

         As long as the law is obligatory, so long our obedience
         is due.                                  --Jer. Taylor.

Source : WordNet®

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"