obligatory

adjective

1. required as a matter of obligation; mandatory: A reply is desirable but not obligatory.
2. incumbent or compulsory (usually followed by on or upon): duties obligatory on all.
3. imposing moral or legal obligation; binding: an obligatory promise.
4. creating or recording an obligation, as a document.

adjective

1. required to be done, obtained, possessed, etc
2. of the nature of or constituting an obligation

adj.c.1400, from Old French obligatoire “creating an obligation, obligatory,” and directly from Late Latin obligatorius “binding,” from obligat-, past participle stem of obligare (see oblige).