In the game of squash, the nick refers to the intersection of the floor with one of the side walls, or with the back wall. Thus there are two side-wall nicks, and one back-wall nick.
The nick is an important part of the squash court as it can make the ball do strange things. A ball that lands exactly in the nick will not bounce out - instead it rolls across the floor. If you play such a shot you should keep a poker face as you calmly fetch the ball. Let your opponent think you can do it on your ear.
A ball that almost lands in the nick is still a very good shot, for two reasons. Firstly it will bounce awkwardly. Secondly, it will cause uncertainty as to whether it will hit the wall or the floor first. If it hits the wall first it will bounce low and fast. If it hits the floor first, it will bounce high and slow. Preparing for both eventualities will be very difficult for your opponent.
Always aim for the nick.