Leader

In everyday freshwater fishing a leader is a two to four foot piece of fluorocarbon joined to braid, connected with an FG knot, a double uni or a small swivel. The braid gives you distance, thin diameter and zero stretch; the leader gives you invisibility where it counts and something tougher than braid against rock and teeth.

In fly fishing a leader is a different object: a tapered clear section, usually seven to nine feet, that transfers the energy of the fly line down to the fly so it turns over and lands straight instead of collapsing in a pile.

Leader length is a compromise. Long enough that the fish never sees the main line, short enough that the connecting knot does not have to travel through the rod guides on every cast.