(1):

(n.) That which is inserted or set in; an insertion.

(2):

(n.) One or more separate leaves inserted in a volume before binding; as: (a) A portion of the printed sheet in certain sizes of books which is cut off before folding, and set into the middle of the folded sheet to complete the succession of paging; - also called offcut. (b) A page or pages of advertisements inserted.

(3):

(v. t.) To infix.