Webster's Revised Unabridged Dictionary · 1913

Interjection

Interjection , noun

[Latin interjectio: compare French interjection. See Interject.]

1.
The act of interjecting or throwing between; also, that which is interjected.
The interjection of laughing. — Bacon
2.
(Grammar) A word or form of speech thrown in to express emotion or feeling, as O! Alas! Ha ha! Begone! etc. Compare Exclamation.
An interjection implies a meaning which it would require a whole grammatical sentence to expound, and it may be regarded as the rudiment of such a sentence. But it is a confusion of thought to rank it among the parts of speech. — Earle
How now! interjections? Why, then, some be of laughing, as, ah, ha, he! — Shakespeare