noun
plural reins
[count] a strap that is fastened to a device (called a bridle) placed on the head of an animal (such as a horse) and that is used to guide and control the animal - usually plural
the ability to limit or control something
the power to guide or control someone or something - usually plural
free/full rein
the opportunity to act freely
verb
reins; reined; reining
rein in
[phrasal verb]
rein (someone or something) in or rein in (someone or something)
to limit or control (someone or something)
to make (an animal) stop by using reins