What is a marker in ultimate Frisbee?

Answer 1

The marker is the proper term for the paintball "gun".

It is called a marker because it "marks" what it shoots with paintballs with a gas propellant instead of a "gun" which propels a metal objects with a small controlled explosion.

Answer 2

A marker is a defender who guards the offender that is in possession of the disc. The marker also yells the stall count and if the count reaches ten before the disc is thrown, then possession is changed.