Antonyms

Definitions

adj. 1
Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid some penalty.
It is absolutely necessary that you call and confirm your appointment.
adj. 2
Unavoidable, inevitable.
If it is absolutely necessary to use public computers, you should plan ahead and forward your e-mail to a temporary, disposable account.
n. 1
A place to do the "necessary" business of urination and defecation: an outhouse or lavatory.

Word family: necessarian necessarily necessariness necessitate necessity
Phrases: necessary condition necessary evil necessary house necessary place necessary stool necessary vault

or Necessary?

That is necessary which must exist, occur, or be true; which in the nature of things can not be otherwise.

J. C. Fernald, English Synonyms and Antonyms (1914)

needed Of course, a license is needed to operate a crane.
required I'm sorry, sir, but a jacket and tie are required.
essential Selfishness is an essential part of his character.
requisite I won't be able to go travelling until the requisite visa fees are paid.
certain It is certain that Lucy will marry the pop singer.
inevitable The Foreign Minister said that war was inevitable.

Use in printed books, per decade from 1850 to 2019.

3 letters: can¹, loo¹
4 letters: head¹, john¹, main¹
5 letters: vital², fatal², privy², jakes¹
6 letters: office², needed², toilet², johnny², closet², urinal
7 letters: certain², natural³, crucial², fateful², needful², latrine², crapper
8 letters: required³, bathroom², integral³, lavatory⁴, washroom², outhouse²
9 letters: important³, essential³, necessity⁴, incumbent³, requisite³
10 letters: inevitable⁵, compulsory⁴, obligatory⁵, infallible⁴, inexorable⁵
11 letters: requirement³, convenience³, unavoidable⁵, involuntary⁵, inescapable⁵, ineluctable, necessitous
12 letters: prerequisite⁴
13 letters: indispensable⁵, unpreventable

The small number after each word is its number of syllables (vital² = 2 syllables).