a tomb: a grave or rock-cut burial chamber, named as the lasting marker of the person laid there
The word belongs to the family of Greek words for remembering, and it names the thing that keeps a dead person in mind: the tomb. In ordinary use that memory idea has faded into the background, and the word simply means the place where a body is laid, whether a cave, a grave, or a chamber cut into rock and sealed with a stone.
All eight New Testament uses point to a real burial place. Luke says the body of Jesus was laid in a tomb cut from rock where no one had ever lain, and that the women came to that tomb at first light. Stephen speaks of the tomb Abraham bought at Shechem. Peter, preaching at Pentecost, can point to the tomb of David as a landmark still standing in Jerusalem, which is his proof that David was speaking of someone other than himself. In Revelation the watching crowds refuse to let the bodies of the two witnesses be put in a tomb, an insult that would have been felt as one of the worst. And in Mark and Luke the tombs outside the Gerasene town are a plural place, a hillside of burial caves, and that is where the man nobody could bind makes his home, night and day among the dead.
Greek had a second and far more common word for the same thing, μνημεῖον, built from the same root, and the two are close to interchangeable. Luke uses this word for the tomb of Jesus in one verse and that other word for the very same tomb two verses later, and some manuscripts swap one for the other. A third word, τάφος, comes instead from the verb for burying, so it leans on the act of burial rather than on the marker left standing.