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Trail
From WikiPedalia
Trail is the distance from the center of the contact point of the front wheel with the riding surface to the intersection of the steering axis (head tube) with the surface. The trail is a function of the head angle, the fork rake, and the tire diameter. Trail has a major effect on the handling of a bicycle. More trail increases the bicycle's tendency to steer straight ahead. A bicycle with a largish trail dimension will be very stable, and easy to ride "no hands". A bicycle with a smaller trail dimension will be more manuverable and responsive.
Joshua Putnam has a good discussion of trail and Bicycle Steering Geometry in general on his Web site.
