The definition of a restraint is something that restricts freedom or prevents someone from doing something.(noun)
See restraint in Webster's New World College Dictionary
Origin: ME restreinte < OFr restrainte < restreindre, to restrain
See restraint in American Heritage Dictionary 4
Origin: Middle English restreinte
Origin: , from Old French restrainte
Origin: , from feminine past participle of restraindre, to restrain; see restrain.
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