Rein (noun)

1. A strap attached to the bit of a bridle, by which a rider or driver controls a horse or other animal.
2. Control or direction, especially as exerted through a means of restraint.

Origin:

Middle english: from old french rene, from latin retinaculum "restraint", from retinere "hold back".

Examples:

1. He pulled on the reins to slow down the horse.
2. She loosened the reins to give the horse more freedom.
3. The government has tightened the reins on spending.
4. The company's new management is putting the reins on excessive risk-taking.
5. He had to take the reins of the project after the lead researcher left.