Rein \Rein\ (r?n), n. [F. r[^e]ne, fr. (assumed) LL. retina, fr. L. retinere to hold back. See {Retain}.] 1. The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse. [1913 Webster] This knight laid hold upon his reyne. --Chaucer. [1913 Webster] 2. Hence, an instrument or means of curbing, restraining, or governing; government; restraint. ``Let their eyes rove without rein.'' --Milton. [1913 Webster] {To give rein}, {To give the rein to}, to give license to; to leave withouut restrain. {To take the reins}, to take the guidance or government; to assume control. [1913 Webster]