rein - noun

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

any means of control

rein - verb

• control and direct with or as if by reins
• stop or slow up one's horse or oneself by or as if by pulling the reins
• stop or check by or as if by a pull at the reins
• keep in check

rein - thesaurus

harness   rule