[sdiy] Rails

[JP]

Hello - I was hoping that someone could explain what the term "rail/rails"
means in regards to electronics.  I've picked up that it has to do w/ power
or power supplies but that's about it.  A couple examples from recent posts:

"5V at the rails"

"rail to rail amplifiers"

"As for current delivery, most will do a few milli-amps, but typically not
near the rails"

"an opamp with a symmetric output swing at each rail"

"as long as your supply rails were properly adjusted."

"This pretty much slams the square right up to the rails"

Thanks,

JP