pastie

pastie /pay'stee/ n.  An adhesive-backed label designed to
   be attached to a key on a keyboard to indicate some non-standard
   character which can be accessed through that key.  Pasties are
   likely to be used in APL environments, where almost every key is
   associated with a special character.  A pastie on the R key, for
   example, might remind the user that it is used to generate the
   rho character.  The term properly refers to
   nipple-concealing devices formerly worn by strippers in concession
   to indecent-exposure laws; compare {tits on a keyboard}.