The Collaborative International Dictionary of English v.0.48:

Taper \Ta"per\, v. i. [imp. & p. p. Tapered; p. pr. & vb. n.
   Tapering.]
   To become gradually smaller toward one end; as, a sugar loaf
   tapers toward one end.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tapering \Ta"per*ing\, a.
   Becoming gradually smaller toward one end. --
   Ta"per*ing*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tapering
    adj 1: becoming gradually narrower; "long tapering fingers";
           "trousers with tapered legs" [syn: tapered, tapering,
           narrowing]
    2: gradually decreasing until little remains [syn: dwindling,
       tapering, tapering off]
    n 1: the act of gradually lowering the size or amount; "the
         doctor prescribed the tapering of the dose"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "tapering":
   acanthoid, acanthous, acicular, acuate, aculeate, aculeiform,
   acuminate, acute, barbed, coarctation, cone-shaped, constriction,
   contraction, cornuted, cusped, cuspidate, hispid, horned, horny,
   mucronate, narrowing, needle-pointed, needle-sharp, pointed,
   pronged, sharp-pointed, spiculate, spiked, spiky, spined, spinous,
   spiny, stricture, taper, tapered, tined, toothed, unbated