From the second edition (1989):
(ɪnfɪkˈseɪʃən) [f. infix v. + -ation.]

The action of infixing; the state of being infixed.

1921 E. Sapir Lang. iv. 76 A peculiarly interesting type of infixation is found in the Siouan languages, in which certain verbs insert the pronominal elements into the very body of the radical element. 1964 E. Bach Introd. Transformational Gram. iii. 48 And the infixation of the object marker‥should be brought about by a low-level rule in the grammar.