OXFORD DICTIONARY
It, abbr. information technology.
It, pron. (poss. its; pl. they)
1 the thing (or occas. the animal or child) previously named or in question (took a stone and threw it).
2 the person in question (Who is it? It is I; is it a boy or a girl?).
3 as the subject of an impersonal verb (it is raining; it is winter; it is Tuesday; it is two miles to Bath).
4 as a substitute for a deferred subject or object (it is intolerable, this delay; it is silly to talk like that; I take it that you agree).
5 as a substitute for a vague object (brazen it out; run for it!).
6 as the antecedent to a relative word (it was an owl I heard).
7 exactly what is needed (absolutely it).
8 the extreme limit of achievement.
9 colloq. sexual intercourse; sex appeal.
10 (in children's games) a player who has to perform a required feat, esp. to catch the others.
that's it colloq. that is:
1 what is required.
2 the difficulty.
3 the end, enough.
this is it colloq.
1 the expected event is at hand.
2 this is the difficulty.
It, n. colloq. Italian vermouth (gin and it).
abbr.
THESAURUS
It
I, I myself, alter, alter ego, alterum, better self, ego, ethical self, he, her, herself, him, himself, inner man, inner self, me, my humble self, myself, not an illusion, number one, oneself, other self, ourselves, self, she, subconscious self, subliminal self, superego, the article, the genuine article, the goods, the very model, the very thing, them, themselves, they, you, yours truly, yourself, yourselves