Tacit describes an agreement, an understanding, approval, or support that is real but was never actually said out loud. Everyone involved knows it, but nobody put it into words. If your boss sees you leaving early every Friday and says nothing for a year, you have her tacit approval — she never said 'yes, go ahead', but her silence has done the work of a yes.

The important part is that tacit does not just mean 'unspoken'. It means unspoken AND understood by both sides. Something can be silent and mean nothing; tacit means the silence itself carries the message. That is why it goes so naturally with a small set of nouns: tacit agreement, tacit approval, tacit understanding, tacit support, tacit consent, tacit admission. You will meet it in those phrases far more often than anywhere else — it is not a word people spread freely over other nouns.

It feels formal and a little cool. You read it in newspapers, in law, in business writing, in academic work. Two friends will not say 'we had a tacit agreement'; they will say 'we just kind of knew'. When a journalist writes that a government gave tacit support to something, there is often a quiet edge to it — the suggestion that the support was deliberately left unspoken so that nobody could be held responsible for it later.

There is one place where it stretches beyond agreements: 'tacit knowledge' — the things an experienced person knows how to do but cannot explain in words, like how a good cook knows when the oil is hot enough. That use comes from philosophy and business writing and is now fairly common.

Only the adjective form exists; there is no verb.