If you jog every morning or run with your school's cross country team, you're a runner. A runner is a person who runs.
A casual runner jogs around the block or chases their dog on the beach. If you train to run in the Olympics, you're also a runner, and if you play baseball, you're a runner when you're moving from first to second base. Plants send out something also called a runner — a new shoot that sits on top of the ground and roots itself there. A strip of carpet down the hall or stairs is yet another kind of runner.