The Collaborative International Dictionary
Interlineation \In`ter*lin`e*a"tion\, n. [Cf. F. interlin['e]ation.]
The act of interlining.
That which is interlined; a passage, word, or line inserted between lines already written or printed.
Wiktionary
n. A notation made between the lines, especially in a handwritten document
Wikipedia
Interlineation is a legal term that signifies writing has been inserted between earlier language. It is commonly used to indicate the insertion of new language between previous sentences in a contract, though it may also be used in other contexts. For example, interlineations can help to clarify specific points of a legal contract, and are useful if some of the terms of a contract become unclear later. They are typically initialed and dated, so as to make a record of changes and agreements to a particular contract.