The Collaborative International Dictionary of English v.0.48:

Impede \Im*pede"\, v. t. [imp. & p. p. Impeded; p. pr. & vb.
   n. Impeding.] [L. impedire, lit., to entangle the feet;
   pref. im- in + pes, pedis, foot. See Foot, and cf.
   Impeach.]
   To hinder; to stop in progress; to obstruct; as, to impede
   the advance of troops.
   [1913 Webster]

         Whatever hinders or impedes
         The action of the nobler will.           --Logfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impede
    v 1: be a hindrance or obstacle to; "She is impeding the
         progress of our project" [syn: impede, hinder]
    2: block passage through; "obstruct the path" [syn: obstruct,
       obturate, impede, occlude, jam, block, close up]
       [ant: disengage, free]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "impede":
   arrest, backpedal, backwater, bar, block, bottle up, brake, burden,
   check, clip the wings, confine, confound, countercheck, cramp,
   cripple, cumber, curb, dam, dam up, damp, dampen, decelerate,
   delay, detain, discomfit, disconcert, drag, draw rein, ease off,
   ease up, embarrass, encumber, enmesh, ensnarl, entangle, entoil,
   entrammel, entrap, entwine, faze, fetter, foil, hamper, hamstring,
   handicap, hinder, hobble, hold back, hold in check, hold up,
   inhibit, intercept, interfere, intermeddle, interrupt, intervene,
   involve, keep back, keep in check, lag, lame, let down, let up,
   lime, lose ground, lose momentum, lose speed, lumber, make late,
   meddle, moderate, net, obstruct, oppose, press down, rattle, reef,
   rein in, relax, repress, resist, restrain, retard, saddle with,
   scotch, set back, shackle, slack off, slack up, slacken, slow,
   slow down, slow up, snarl, snub, spike, stay, stop, suppress,
   take in sail, tangle, throttle down, thwart, toil, trammel,
   weigh down