"X, Y, Z Receivers
The x receiver, or the split end, aligns on the weak side of the formation. The z receiver, or the flanker, aligns on the strong side of the formation, maybe a couple of steps off the line of scrimmage in the slot. The tight end functions as the y receiver, but often on passing plays functions as another wide receiver."
This is the main definition of a WR. But my explanation is that the #1 WR is the player, who is the best WR from a group of wide recievers on a team, either because of his leadership hands, speed, or route running. It's confusing, but it makes sense.