The Collaborative International Dictionary of English v.0.48:

Post-it note \Post"-it note\, n. [From the trademark Post-it
   registered by the 3M Corporation.]
   A small sheet of paper having the back part partly covered
   with a non-permanent gum which allows the note to be attached
   temporarily to another object, and easily removed without
   leaving any trace of glue on the object to which it was
   affixed. Such note papers are sold in pads of varying sizes.
   [Trademark]
   [PJC]