[1] : WordNet (r) 2.0

add-on
     n 1: a component that is added to something to improve it; "the
          addition of a bathroom was a major improvement"; "the
          addition of cinammon improved the flavor" syn: addition,
           improver
     2: a supplementary component that improves capability syn: accessory,
         appurtenance, supplement
see also:
addition improver accessory appurtenance supplement 

[2] : Easton's 1897 Bible Dictionary

addon
   low, one of the persons named in neh. 7:61 who could not "shew
   their father's house" on the return from captivity. this, with
   similar instances ver. 63, indicates the importance the jews
   attached to their genealogies.
   

[3] : Hitchcock's Bible Names Dictionary (late 1800's)

addon, basis; foundation; the lord

Add-On >> A Words
Add-On, definition of term: Add-On
add-on_pag1.html