The lateral distance between the pedal attachment points on a crankset. For most cyclists, the narrower the tread the better. When we walk, our footprints fall almost in a straight line under the center-of-gravity of our bodies. When we cycle, the "tread" forces our feet to be exerting their force farther apart than they are designed to do. Some theorists blame this for knee and hip problems experienced by some cyclists, but the jury is still out on this.
A wider "tread" also creates a greater sideways force when pedaling out of the saddle, so the rider must exert more upper-body strength when riding "en danceuse".
A wider "tread" decreases pedal-ground clearance when the bicycle is banked over into a tight turn.
("Tread" in this sense is sometimes referred to as "Q-factor.")