Refers to changes in wheel alignment (toe, camber and caster) as the wheel moves through the suspension range. Wheel alignment is set with the car stationary, so bump steer affects must be properly considered to ensure that suspension movement does not cause adverse changes in handling or grip.
The ill-effect a fully compressed damper has on steering through turns. Referred to as such because often times when a damper is fully compressed, it is against its bump stop. When this point is reached, neither the damper nor the spring can do it's job, usually causing loss of grip. May also refer to when the wheels steer themselves, without input from the driver, caused by bumps in the track interacting with improper suspension tuning or alignment.