adjective (a)

• impossible or difficult to perceive by the mind or senses(adj.all)

  Example:
  An imperceptible drop in temperature.
  An imperceptible nod.
  

  Synonym:
  unperceivable
  Antonym:
  perceptible
  

  source: wordnet30
• Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended.(adjective)

  source: webster1913