Simple Definition of constrain : to limit or restrict (something or someone) : to use pressure to force (someone) to do something Simple Definition of restrain : to prevent (a person or animal) from doing something : to prevent (a person or animal) from moving by using physical force : to keep (something) under control Simple Definition of restrict : to limit the amount or range of (something) : to prevent (someone) from doing something : to allow (someone) to only have or do a particular thing