Synonyms for Addition (same or very similar meaning)
WordNet sense 1 (a quantity that is added):
gain, increase, increase
WordNet sense 2 (something added to what you already have):
accession
WordNet sense 3 (a component that is added to something to improve it):
add-on, improver
WordNet sense 4 (the arithmetic operation of summing; calculating the sum of two or more numbers):
plus, summation
From the ODE community, based on WordNet
add/edit