rein vs rein in

rein is a noun but rein in is not a noun.

Word	Noun	Adjective	Verb	Adverb
rein	Yes	No	Yes	No
rein in	No	No	Yes	No

As verbs, rein and rein in are synonyms defined as:

rein and rein in: stop or slow up one's horse or oneself by or as if by pulling the reins

As verbs, rein and rein in are synonyms defined as:

As verbs, rein and rein in are synonyms defined as:

Other synonyms of rein include draw rein, harness.

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