I'm going to start reining and even I don't know how to define it!! Let's check wikipedia...
"Reining is a western riding competition for horses where the riders guide the horses through a precise pattern of circles, spins, and stops. All work is done at the lope (known more commonly worldwide as the canter) and gallop; the fastest of the horse gaits. Reining is often described as a Western form of dressage riding, as it requires the horse to be responsive and in tune with its rider, whose aids should not be easily seen, and judges the horse on its ability to perform a set pattern of movements." -Wikipedia
Now we know :D