An O'Neill Cylinder is an at present theoretical space based mega structure. Imagine you wanted to live in outer space. There are a lot of considerations. You'll want an enclosed space that you can fill with air so you don't have to spend every moment in a space suit, a source of food and water, and you'll probably want some way of simulating gravity so your bones don't degrade. Any air tight container that you can fit inside will work but the bigger the better for the purpose of both legroom and having a lot of air in it. Gravity is a bit trickier but a reasonable substitute can be implemented by centrifugal force. What's a shape that you can spin and gives a large area where all that area will experience the same outward force? Yep, cylinder.
While multiple people conceived of the rotating habitats before him, Gerard K. O'Neill proposed this idea in his 1976 book The High Frontier: Human Colonies in Space and his name stuck. The design proposed in the book has a pair of five mile wide, twenty mile long tubes divided into six equal strips. Three of the strips are land area where you can put buildings and parks and the other three are windows that allow giant mirrors to reflect light in for day time and can be closed to create night. A picture is worth a thousand words. While the classic O'Neill Cylinders are pretty cool I'd argue that there is a lot of optimizations to be had. Lose the windows to get more surface area and get the light from outside with a more complicated arrangement of mirrors. Add a ceiling and a vacuum boundary and nest cylinders inside one another Matryoshka doll style for even more living area. The nesting would cost you the psuedo-sky in most of the cylinders but you're already living in a bottle so ... eh.
O'Neill Cylinders show up in a handful of science fiction settings such as Gundam, Babylon 5, and Interstellar. I consider this a fairly small list given how much more living area they provide per ton of matter. As I've mentioned in previous works, I find the notion of whole sale terraforming planets rather dubious, not from a plausibility stand point, it's obviously possible given advanced enough infrastructure, but from a simple economy. Building an O'Neill Cylinder would be a gargantuan undertaking but still way less than say making Mars livable and with a much faster return on investment. Combine that with the lower gravity and reduced solar output and I'd much rather be moving mass off of the moon and into Lagrange points than trying to get to dusty old Mars. Not that we can't do both.
IRON NODER XIV: THE RETURN OF THE IRON NODER