a lake: still inland water ringed by land, large enough to sail across but not the open sea
This is the ordinary Greek word for a lake: standing water with land all the way around it, big enough for boats but nothing like the ocean. The New Testament uses it for only two things, and they could hardly be further apart. Luke uses it for the water at the center of Jesus' Galilean ministry, and Revelation uses it for the burning lake into which evil is finally thrown.
Luke is the only Gospel writer who calls the Galilee water a lake. Matthew, Mark, and John call it a sea (θάλασσα), following the habit of Hebrew, whose word for sea (יָם) covers any large body of water at all. Luke simply reaches for the more exact Greek term. Nothing theological hangs on the difference: it is the same stretch of fresh water in both accounts, and it produces the same sudden, terrifying squalls.
In Revelation the same plain word turns terrible. Four scenes throw something into the lake of fire, and what goes in widens each time:
- the beast and the false prophet, thrown in alive
- the devil who had deceived the nations
- death itself and the realm of the dead
- everyone whose name is missing from the book of life
The force of the image depends on the word being ordinary. A lake has edges and a bottom, and whatever goes in does not wash back out. It stands apart from the bottomless deep (ἄβυσσος), which in Revelation is a prison that gets opened and sealed and opened again. The lake of fire is where things end.