Twi- (A.S.), double, as in twilight. [A.S. twí-, double, twá, two.]
U- (Gr.), no, not, as Utopia. [Gr. ou, not.]
Ultra- (L.), beyond, as ultramarine. The French form outre appears in outrage and in utterance. [From ulter (stem of ulterior), ul- being from root of L. ille.]
Um-, in umpire. See Non-.
Un- (A.S.), negative prefix, not, as unhappy, untruth, uncouth. [Cog. with Gr. an- and L. in- (negative).]
Un- (A.S.), verbal prefix, signifying the reversal of an action, as unlock, unbind, undo, unwind. [A.S. on-, un-; cf. Dut. ont-, Ger. ent-, Goth. and-. See A- (A.S.) (2).]
Un-, Uni- (L.), one, as unanimous, uniform. [L. unus, one.]
Under- (A.S.), under, below, as undergrowth, underwood, underprop, undersell. [See Under in Dict.]
Up- (A.S.), up, as upland, upstart, upright, uphill, upbraid, upset. [A.S. up, upp; Ger. auf.]
Ve- (L.), apart from, as vestibule. [L. ue, apart from; prob. allied to bi- and duo, two.]