Nothing anymore. Originally it measured a specific dimension on the transistor. Then transistors started shrinking non-uniformly, so they adjusted it -- "how b

Nothing anymore. Originally it measured a specific dimension on the transistor. Then transistors started shrinking non-uniformly, so they adjusted it -- "how big the transistors would be at equivalent density if they had shrunk evenly". That's an easy thing to play with though, and so you see more and more departures from reality.