a grave: the place where a dead body is laid, named from the act of burying it
Greek had more than one word for a grave, and this one is built on the verb for burying (θάπτω), so it names the place by what happens there: a body is put in and left. It covers a chamber cut into rock and closed with a stone, and it covers a tomb built up above ground where a family or a town keeps the memory of its dead. Six of its seven uses in the New Testament are in Matthew.
Four of those six are the tomb of Jesus, and they come at the point in the story where the tomb stops being a resting place and becomes a guarded site. A watch is posted, the stone is sealed, two women sit facing it, and the same two come back at first light to look at it. Matthew had used the commoner Greek word for a tomb (μνημεῖον) one sentence earlier for the very same rock chamber, so the two words are close to interchangeable in his hands.
Matthew's other two uses turn the word against the people it describes. Building and keeping up a grave is an act of honor, so the leaders who raise buildings over the graves of the prophets are doing what a grateful people does. But in the same speech they are compared to graves that have been coated with lime: handsome from the road, and packed with bones and decay behind the plaster. Both halves of that charge depend on a grave being a thing you can see and tend from the outside while its contents stay hidden.
Paul's single use, quoting a psalm, keeps the picture and moves it onto a person. A man's throat is a grave standing open, so that what comes out of his mouth is what comes out of a grave.