Definitions 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

  Example: "He took up the reins of government"

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

  Example: "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

  Example: "They reined in in front of the post office"

• verb:   keep in check
• verb:   control and direct with or as if by reins

  Example: "Rein a horse"

• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #11968)