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Vine's Expository Dictionary of NT Words [1]

1: Ἀναγκαῖος (Strong'S #316 — Adjective — anankaios — an-ang-kah'-yos )

"necessary" (from ananke, "necessity;" see below), is so rendered in  Acts 13:46;  1—Corinthians 12:22;  2—Corinthians 9:5;  Philippians 2:25;  Titus 3:14;  Hebrews 8:3 , RV (AV, "of necessity"); for  Acts 10:24 , "near friends," see Near , B.

2: Ἀνάγκη (Strong'S #318 — Noun Feminine — ananke — an-ang-kay' )

"a necessity" (see No. 1), is rendered "(it was) necessary" in  Hebrews 9:23 , lit., "it was a necessity." See Distress , A, No. 1.

3: Ἐπάναγκες (Strong'S #1876 — Adjective Neuter — epanankes — ep-an'-ang-kes )

an adjective akin to the preceding, with epi, used intensively, found only in the neuter form, is used as an adverb signifying "of necessity" and translated as an adjective in  Acts 15:28 , "necessary," lit., "(things) of necessity."

 Acts 28:10Need

Webster's Dictionary [2]

(1): ( a.) Acting from necessity or compulsion; involuntary; - opposed to free; as, whether man is a necessary or a free agent is a question much discussed.

(2): ( a.) Impossible to be otherwise, or to be dispensed with, without preventing the attainment of a desired result; indispensable; requiste; essential.

(3): ( a.) Such as must be; impossible to be otherwise; not to be avoided; inevitable.

(4): ( n.) A thing that is necessary or indispensable to some purpose; something that one can not do without; a requisite; an essential; - used chiefly in the plural; as, the necessaries of life.

(5): ( n.) A privy; a water-closet.

(6): ( n.) Such things, in respect to infants, lunatics, and married women, as are requisite for support suitable to station.

King James Dictionary [3]

Necessary a.

1. That must be that cannot be otherwise indispensably requisite. It is necessary that every effect should have a cause. 2. Indispensable requisite essential that cannot be otherwise without preventing the purpose intended. Air is necessary to support animal life food is necessary to nourish the body holiness is a necessary qualification for happiness health is necessary to the enjoyment of pleasure subjection to law is necessary to the safety of persons and property. 3. Unavoidable as a necessary inference or consequence from facts or arguments. 4. Acting from necessity or compulsion opposed to free. Whether man is a necessary or a free agent is a question much discussed.

NECESSARY, n. A privy.