1) Binding 2) Bounden 3) Compulsory 4) Crucial 5) Essential 6) Imposed 7) Inescapable 8) Mandatory 9) Necessary 10) Needed 11) Prerequisite 12) Required 13) Unthinking 1) Bounden 2) Compulsory 3) De rigueur 4) Imposed 5) Incumbent on 6) Indispensable 7) Mandatory 8) Necessary 9) Prerequisite 10) Required 
• (a.) 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.
obligate or required. For example, an obligatory cleaner fish relies entirely on this feeding mode to obtain nutrients. 
(әb-lig´ә-tor″e) obligate.
Ob'li·ga·to·ry adjective [ Latin obligatorius : confer French 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. &#x...
Not optional.
[adj] - required by obligation or compulsion or convention 2. [adj] - morally or legally constraining or binding
obligatory 1. Required by obligation or compulsion or convention. 2. Morally or legally constraining or binding.
adjective required by obligation or compulsion or convention; `he made all the obligatory apologies`