The Collaborative International Dictionary of English v.0.48:

Diversion \Di*ver"sion\, n. [Cf. F. diversion. See Divert.]
   1. The act of turning aside from any course, occupation, or
      object; as, the diversion of a stream from its channel;
      diversion of the mind from business.
      [1913 Webster]

   2. That which diverts; that which turns or draws the mind
      from care or study, and thus relaxes and amuses; sport;
      play; pastime; as, the diversions of youth. "Public
      diversions." --V. Knox.
      [1913 Webster]

            Such productions of wit and humor as expose vice and
            folly, furnish useful diversion to readers.
                                                  --Addison.
      [1913 Webster]

   3. (Mil.) The act of drawing the attention and force of an
      enemy from the point where the principal attack is to be
      made; the attack, alarm, or feint which diverts.

   Syn: Amusement; entertainment; pastime; recreation; sport;
        game; play; solace; merriment.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diversion
    n 1: an activity that diverts or amuses or stimulates; "scuba
         diving is provided as a diversion for tourists"; "for
         recreation he wrote poetry and solved crossword puzzles";
         "drug abuse is often regarded as a form of recreation"
         [syn: diversion, recreation]
    2: a turning aside (of your course or attention or concern); "a
       diversion from the main highway"; "a digression into
       irrelevant details"; "a deflection from his goal" [syn:
       diversion, deviation, digression, deflection,
       deflexion, divagation]
    3: an attack calculated to draw enemy defense away from the
       point of the principal attack [syn: diversion,
       diversionary attack]

Moby Thesaurus II by Grady Ward, 1.0:

237 Moby Thesaurus words for "diversion":
   aberrancy, aberration, about-face, abuse, abuse of office,
   accommodation, adaptation, adjustment, aerial tactics,
   aggravated assault, aggression, airborne tactics, alteration,
   amelioration, amphibious attack, amusement, apostasy,
   applied tactics, armed assault, armored tactics, assailing,
   assailment, assault, attack, banzai attack, barrier tactics,
   befoulment, bend, betterment, bias, blitz, blitzkrieg,
   branching off, break, breakthrough, bypass, cavalry tactics,
   change, change of heart, changeableness, charge, circuitousness,
   constructive change, continuity, conversion, corner,
   corrupt administration, counterattack, counteroffensive,
   coup de main, crippling attack, crook, curve, dead set at,
   debasement, declination, defalcation, defection,
   defensive strategy, defilement, deflection, degeneration,
   degenerative change, delectation, departure, descent on,
   desecration, deterioration, detour, deviance, deviancy, deviation,
   deviousness, difference, digression, discontinuity, discursion,
   disport, dissipation, distractedness, distraction, divagation,
   divarication, divergence, diversification, diversionary attack,
   diversity, divertisement, divertissement, divided attention,
   dogleg, double, drift, drifting, drive, embezzlement, encirclement,
   enjoyment, entertainment, errantry, excursion, excursus,
   exorbitation, feint, fire tactics, fitting, flank attack,
   flip-flop, fouling, frivolity, frontal attack, fun, game,
   gas attack, gradual change, hairpin, head-on attack, improvement,
   inattention, indirection, infantry tactics, infiltration,
   investment, levity, lightning attack, lightning war,
   maladministration, malfeasance, malpractice, malversation,
   mass attack, megadeath, melioration, mirth, misapplication,
   misappropriation, misconduct, misemployment, misfeasance,
   mishandling, mismanagement, misusage, misuse, mitigation,
   mob tactics, mobile tactics, modification, modulation, mugging,
   obliquity, offense, offensive, onset, onslaught, overkill,
   overthrow, panzer warfare, passe-temps, pastime, peculation,
   pererration, perversion, pilfering, pincers movement, play,
   pleasure, pollution, poor stewardship, profanation, prostitution,
   push, qualification, radical change, rambling, re-creation,
   realignment, recreation, redesign, reform, reformation, regalement,
   relaxation, relish, remaking, renewal, reshaping, restructuring,
   reversal, revival, revivification, revolution, run against, run at,
   rush, sally, sheer, shift, shifting, shifting course,
   shifting path, shock tactics, sidetrack, skew, slant, solace,
   sortie, sport, strategy, straying, strike, sudden change, sweep,
   swerve, swerving, swinging, switch, tack, tactics, total change,
   transition, turn, turnabout, turning, twist, unprovoked assault,
   upheaval, variation, variety, veer, violation, violent change,
   wandering, warp, withdrawal of attention, worsening, yaw, zigzag