1. appurtenance An adjunct (something joined to) constitutes no real part of the thing or system to which it is joined, tho perhaps a valuable addition; an appendage is commonly a real, tho not an essential or Synonyms: appendage, accessory, addition, appurtenance, concomitant, accompaniment, adjunct, attachment, extension, addendum, appendix, auxiliary, supplement Antonyms: main body, original, total, whole