A horse's speed between trotting and galloping — comfortably fast, but not the horse's hardest running. As a verb, either the horse canters, or the rider canters (rides at that speed), or the rider canters the horse (makes it go at that speed): all three are normal. As a noun you say at a canter, or break into a canter. Riders care about this word because a horse has set gaits — walk, trot, canter, gallop — and each one feels different to sit on; canter is the smooth rocking one. Outside riding, English uses at a canter to mean winning easily, without having to try hard: the team won at a canter. Separately, and confusingly, canter can also be a person who cants — but that is a different word, and you will almost never meet it.