a well: a deep shaft dug straight down into the ground, either to reach water or to open into the depths below
The picture behind this word is a hole dug straight down and walled so it will not collapse. It does not name the water, and it does not name the spring that feeds it: it names the shaft, and above all the depth of the shaft. That is why the woman at Sychar answers Jesus by pointing at the stonework rather than the water, telling him he has nothing to draw with and that the shaft is deep. John can call the same spot a spring (πηγή) and then let the woman call it a well, because one word names water rising on its own and the other names the shaft sunk down to reach it.
A hole that deep is also a hazard, and the New Testament assumes as much. Luke's question about an ox falling in on the Sabbath takes for granted that these openings sat out in open country and that hauling an animal back up was work no one would put off until the next day. Matthew tells a similar story with a plainer word for a hole in the ground (βόθυνος), while this word keeps the sense of something deliberately built and sunk.
Revelation then takes that ordinary object and uses it for something else entirely. The abyss (ἄβυσσος) is pictured as a vast prison under the earth, and the way into it is a shaft with a mouth that can be locked and unlocked. When it is opened, smoke pours up out of it like the smoke of a great furnace. The image works only because everyday readers knew exactly what a well looked like: a narrow opening, a long drop, and no way to see the bottom.