Synonyms of the word

obligatory

• adj. Imposing obligation, morally or legally; binding.
• adj. Requiring a matter or obligation.

bounden

• adj. Under an obligation; obliged (to do something).

compulsory

• adj. Required; obligatory; mandatory.
• adj. Having the power of compulsion; constraining.
• n. Something that is compulsory or required.

imposed

• v. simple past tense and past participle of impose.

indispensable

• adj. (ecclesiastical, obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption;…
• adj. (of duties, rules etc.) Unbendable, that cannot be set aside or ignored.
• adj. Absolutely necessary or requisite; that one cannot do without.
• n. A thing that is not dispensable; a necessity.
• n. (in the plural, colloquial, dated) Trousers.

mandatory

• adj. Obligatory; required or commanded by authority.
• adj. Of, being or relating to a mandate.
• n. (dated, rare) A person, organisation or state who receives a mandate; a mandatary.

necessary

• adj. Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid…
• adj. Unavoidable, inevitable.
• adj. (obsolete) Determined, involuntary: acting from compulsion rather than free will.
• n. (Britain, archaic euphemistic, usually with the definite article) A place to do the "necessary" business…

prerequisite

• adj. Required as a prior condition of something else; necessary or indispensable.
• n. Something that must be gained in order to gain something else.
• n. In education, a course or topic that must be completed before another course or topic can be started.…

required

• v. simple past tense and past participle of require.
• adj. Necessary; obligatory; mandatory.