Definition: n-key rollover

The number of keyboard keys that can be depressed and held down while all keys input the appropriate character. For example, if pressing A, S and D in sequence without lifting the fingers from all three keys generates ASD on screen, the keyboard has 3-key rollover (3KRO), which is necessary for fast touch typing. Gamers and Braille users generally require 6-key rollover (6KRO).

Test the Rollover
To test the rollover number, press three, then four and then five keys simultaneously. If they all display on screen, the keyboard has rollover for that number of keys.