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"