obligatory

adj. binding, compelling; must be done; necessary

An obligation is a requirement to take some course of action. It can be legal or moral. There are also obligations in other normative contexts, such as obligations of etiquette, social obligations, and possibly the In terms of politics, obligations are requirements that are to fulfill. These are generally in the form of legal obligations, which incur a penalty for lack of fulfillment, although certain people are obliged to carry out certain actions for other reasons as well, which can be due to tradition or social reasons. Obligations vary from person to person, for example, a person holding a political office will generally have far more obligations than an average adult citizen, who themselves will have more obligations than a child. Obligations are generally granted in return for an increase in an individual’s rights or power.
See more at Wikipedia.org...

Adjective
1. morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution"
(antonym) optional
(similar) bounden
(see-also) necessary
2. required by obligation or compulsion or convention; "he made all the obligatory apologies"
(similar) necessary

(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.
  

adj.
ضرور, واجب, لازم, فرض, ناگزير