WordWeb Online

1. Incapable of being perceived by the senses especially the sense of touch
   "the impalpable constituent of energy";
   - intangible
    
2. Imperceptible to the senses or the mind
   "an impalpable cloud"; "impalpable shadows"; "impalpable distinctions"; "as impalpable as a dream"
    
3. Not perceptible to the touch
   "an impalpable pulse"

See also: abstract, elusive, imperceptible, subtle, unperceivable

Antonym: palpable

Encyclopedia: Impalpable