Bump steer is toe change under compression/rebound. If you hit a bump, the toe might go in or out and the car 'steers' a little under cornering.

Zero bumpsteer is an impossible figure, so just try and make sure the geometry you are dealing with has the least amount of toe in/toe out change under the entire range of suspension travel. For example, our AFX spindle on a 1st gen Camaro has thirty-thousandths of an inch of bumpsteer over 4.5 inches of suspension travel.