What does "interjection" mean?

Interjection: (Gram.) 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! Shak.