"Rein in" is a phrase that refers to the act of controlling or restraining someone or something. There are several synonyms that can be used in place of "rein in" including "curb," "limit," "restrict," "constrain," and "moderate." These are all words that suggest the need for caution or control in order to prevent something from getting out of hand. Other synonyms that are less commonly used but still convey a similar meaning include "tether," "bridle," and "harness." Whether it's restraining a horse or managing a difficult situation, the right synonym for "rein in" can help communicate the appropriate level of control needed.