The Collaborative International Dictionary of English v.0.48:

Appreciable \Ap*pre"ci*a*ble\, a. [Cf. F. appr['e]ciable.]
   Capable of being appreciated or estimated; large enough to be
   estimated; perceptible; as, an appreciable quantity. --
   Ap*pre"ci*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

appreciable
    adj 1: enough to be estimated or measured; "appreciable amounts
           of noxious wastes are dumped into the harbor"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "appreciable":
   apparent, appraisable, apprehensible, ascertainable, assessable,
   calculable, clear, cognizable, comprehensible, computable,
   conceivable, concrete, detectable, determinable, discernible,
   discoverable, distinguishable, estimable, evident, fathomable,
   gaugeable, graspable, knowable, manifest, material, measurable,
   mensurable, meterable, noticeable, numerable, observable, obvious,
   palpable, perceptible, plain, ponderable, prehensible,
   quantifiable, quantizable, real, recognizable, seizable, sensible,
   solid, substantial, substantive, tangible, understandable,
   weighable