rein (Noun) — One of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse.
rein (Noun) — Any means of control. ex. "he took up the reins of government"
rein (Verb) — Control and direct with or as if by reins. ex. "rein a horse" ex. "rein in a horse"
rein (Verb) — (driving) stop or slow up one's horse or oneself by or as if by pulling the reins. ex. "They reined in in front of the post office"
rein (Verb) — Stop or check by or as if by a pull at the reins. ex. "He reined in his horses in front of the post office"
rein (Verb) — Keep in check.