The Collaborative International Dictionary of English v.0.48:

Subside \Sub*side"\, v. i. [imp. & p. p. Subsided; p. pr. &
   vb. n. Subsiding.] [L. subsidere; sub under, below + sidere
   to sit down, to settle; akin to sedere to sit, E. sit. See
   Sit.]
   1. To sink or fall to the bottom; to settle, as lees.
      [1913 Webster]

   2. To tend downward; to become lower; to descend; to sink.
      "Heaven's subsiding hill." --Dryden.
      [1913 Webster]

   3. To fall into a state of quiet; to cease to rage; to be
      calmed; to settle down; to become tranquil; to abate; as,
      the sea subsides; the tumults of war will subside; the
      fever has subsided. "In cases of danger, pride and envy
      naturally subside." --C. Middleton.
      [1913 Webster]

   Syn: See Abate.
        [1913 Webster] Subsidence

WordNet (r) 3.0 (2006):

subside
    v 1: wear off or die down; "The pain subsided" [syn: subside,
         lessen]
    2: sink to a lower level or form a depression; "the valleys
       subside"
    3: sink down or precipitate; "the mud subsides when the waters
       become calm" [syn: subside, settle]
    4: descend into or as if into some soft substance or place; "He
       sank into bed"; "She subsided into the chair" [syn: sink,
       subside]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "subside":
   abate, ablate, advance, ascend, back, back up, bate, be eaten away,
   budge, calm, calm down, cave, cave in, cease, change, change place,
   circle, climb, consume, consume away, corrode, crumble, decline,
   decrease, deliquesce, descend, die, die away, die down, diminish,
   dive, droop, drop, drop down, drop off, dwindle, ebb, erode, fade,
   fail, fall, fall away, fall off, flop, flop down, flow, flump,
   flump down, founder, get over, give way, go, go around, go down,
   go downhill, go off, go round, go sideways, gravitate, gyrate,
   halt, head, hit a slump, hit rock bottom, hit the skids, incline,
   languish, lapse, lead, lean, lessen, let up, lower, lull,
   melt away, moderate, molder, mount, move, move over, pacify,
   pass away, plop, plop down, plummet, plump, plunge, point,
   precipitate, progress, quiesce, quiet, quieten, reach the depths,
   recede, regress, retrogress, rise, rotate, run, run down, run low,
   sag, set, settle, settle down, shift, shrink, sink, sink down,
   slacken, slide, slip, slouch, slump, slump down, soar, soothe,
   spin, stir, stop, stream, submerge, swag, tail off, tend,
   tend to go, touch bottom, tranquilize, travel, wane, waste,
   waste away, wear, wear away, wear off, whirl