WordWeb Online

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

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

Antonym: tangible

Encyclopedia: Impalpable