"Aftertouch" is pressure sensitivity, built into the keybed itself. Permits different responses depending on keystroke pressure. Some players swear by it and use it extensively. I do not. But it is respected and many KB players will not use a board that has only velocity settings. More on this and other terms used with controllers, synths, etc. HERE ...

http://www.musiciansnews.com/piano/encyclopedia/92/aftertouch.shtml

PS: the Yamaha KX controller line does include a 25-key model.