The Collaborative International Dictionary of English v.0.48:

Let-up \Let"-up`\ (l[e^]t"[u^]p`), n. [See Let to forbear.]
   Abatement; also, cessation; as, it blew a gale for three days
   without any let-up. [Colloq.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

letup
    n 1: a pause during which things are calm or activities are
         diminished; "there was never a letup in the noise" [syn:
         letup, lull]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "letup":
   abatement, abeyance, abridgment, allayment, alleviation, arrest,
   assuagement, attenuation, blunting, breach, break, breath,
   breather, breathing place, breathing space, breathing spell,
   breathing time, caesura, calming, cease-fire, cessation, check,
   cigarette break, cocktail hour, coffee break, contraction,
   dampening, damping, day off, deadening, deceleration, decrease,
   decrement, decrescence, deduction, deflation, delay, demulsion,
   depreciation, depression, detention, diminishment, diminution,
   downtime, drag, drop, dulcification, dulling, dying, dying off,
   ease-off, ease-up, easing, enforced respite, extenuation, fade-out,
   falling-off, fissure, flagging, gap, half time,
   half-time intermission, halt, happy hour, hesitation, hiatus,
   holdup, holiday, hushing, interim, interlude, intermezzo,
   intermission, intermittence, interregnum, interruption, interval,
   lacuna, lag, languishment, lapse, layoff, leniency, lessening,
   letdown, lightening, loosening, lowering, lull, lulling,
   miniaturization, minus acceleration, mitigation, modulation,
   mollification, obstruction, off-time, pacification, palliation,
   pause, plateau, point of repose, quiet spell, quietening, quieting,
   recess, reduction, relaxation, relief, remission, respite, rest,
   resting point, retardation, retardment, sagging, scaling down,
   setback, simplicity, slack-up, slackening, slowdown, slowing,
   slowing down, slowup, softening, soothing, spell, stand-down, stay,
   subduement, subtraction, surcease, suspension, tea break,
   tempering, time off, time out, tranquilization, truce, vacation,
   weakening