Source : Webster's Revised Unabridged Dictionary (1913)

Impalpable \Im*pal"pa*ble\, a. [Pref. im- not + palpable: cf. F.
   impalpable.]
   1. Not palpable; that cannot be felt; extremely fine, so that
      no grit can be perceived by touch. ``Impalpable powder.''
      --Boyle.

   2. Not material; intangible; incorporeal. ``Impalpable, void,
      and bodiless.'' --Holland.

   3. Not apprehensible, or readily apprehensible, by the mind;
      unreal; as, impalpable distinctions.

Source : WordNet®

impalpable
     adj 1: incapable of being perceived by the senses especially the
            sense of touch; "the intangible constituent of
            energy"- James Jeans [syn: {intangible}] [ant: {tangible}]
     2: imperceptible to the senses or the mind; "an impalpable
        cloud"; "impalpable shadows"; "impalpable distinctions";
        "as impalpable as a dream" [ant: {palpable}]
     3: not perceptible to the touch; "an impalpable pulse"