The Collaborative International Dictionary of English v.0.48:

Flee \Flee\ (fl[=e]), v. i. [imp. & p. p. Fled (fl[e^]d); p.
   pr. & vb. n. Fleeing.] [OE. fleon, fleen, AS. fle['o]n
   (imperf. fle['a]h); akin to D. vlieden, OHG. & OS. fliohan,
   G. fliehen, Icel. fl[=y]ja (imperf. fl[=y][eth]i), Dan. flye,
   Sw. fly (imperf. flydde), Goth. [thorn]liuhan. [root]84. Cf.
   Flight.]
   To run away, as from danger or evil; to avoid in an alarmed
   or cowardly manner; to hasten off; -- usually with from. This
   is sometimes omitted, making the verb transitive.
   [1913 Webster]

         [He] cowardly fled, not having struck one stroke.
                                                  --Shak.
   [1913 Webster]

         Flee fornication.                        --1 Cor. vi.
                                                  18.
   [1913 Webster]

         So fled his enemies my warlike father.   --Shak.
   [1913 Webster]

   Note: When great speed is to be indicated, we commonly use
         fly, not flee; as, fly hence to France with the utmost
         speed. "Whither shall I fly to 'scape their hands?"
         --Shak. See Fly, v. i., 5.
         [1913 Webster]

WordNet (r) 3.0 (2006):

flee
    v 1: run away quickly; "He threw down his gun and fled" [syn:
         flee, fly, take flight]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "flee":
   abscond, absquatulate, avoid, bail out, be annihilated,
   be consumed, be destroyed, be gone, be no more, be wiped out,
   beat a retreat, beat it, blow, bolt, break, break away, break jail,
   break loose, bunk, cease, cease to be, cease to exist, clear out,
   cut and run, cut loose, cut out, decamp, dematerialize, depart,
   desert, die, die away, die out, disappear, dispel, disperse,
   dissipate, dissolve, do a fade-out, dwindle, elope, elude, erode,
   escape, escape prison, eschew, evade, evanesce, evaporate, exit,
   expire, fade, fade away, fade out, fly, fly the coop, fugitate,
   get away, get clear of, get free, get out, get out of, go, go AWOL,
   go away, hide, hightail, jump, jump bail, leave no trace,
   leave the scene, levant, make a getaway, make an exit, make off,
   make tracks, melt, melt away, pass, pass away, pass out, perish,
   peter out, quit, retire from sight, run, run along, run away,
   run away from, run away with, run for it, run off, scamper, scape,
   scoot, scram, screw, show the heels, shun, sink, sink away,
   skedaddle, skip, skip out, slip the cable, slip the collar, split,
   suffer an eclipse, take French leave, take flight, take off,
   take to flight, take wing, turn tail, vamoose, vanish,
   vanish from sight, waste, waste away, wear away