Definition of "Rein" :

noun: one of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse

noun: any means of control

"He took up the reins of government."

verb: keep in check

verb: stop or check by or as if by a pull at the reins

"He reined in his horses in front of the post office."

verb: stop or slow up one's horse or oneself by or as if by pulling the reins

"They reined in in front of the post office."

verb: control and direct with or as if by reins

"Rein a horse."