Interlineation Definition: An addition of something to a document after it has been signed. Such additions are ignored unless they are initialed by the signatories and, if applicable, witnesses (eg. wills). A handwritten addition of something to a typed or otherwise completed document. Such additions are ignored unless they are initialed by the signatories and, if applicable, witnesses (eg. wills). Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!