Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
addĭtĭo, ōnis, f. [addo], an adding to, addition: figurarum additio et abjectio, Quint. 9, 3, 18: Sic corpori fit additio, Cael. Aur. Acut. 2, 37; Prisc. p. 978 P.