Rail-to-Rail
An op-amp typically has a positive and negative voltage supply and, by being rail-to-rail, the output of the op-amp can be all the way from the negative supply voltage to the positive supply voltage. Non rail-to-rail op-amps can only go within a few volts of the input supply.
Rail-to-rail means that the positive maximum is near the positive DC supply voltage and the negative maximum is near the negative DC supply voltage.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd