To prove that something, such as a document, is what it purports to be, especially so that the item can be admitted into evidence at a trial or hearing. For example, a party wishing to admit a letter into evidence may ask the witness whether it is, indeed, the letter he received, does he recognize the handwriting, and similar questions.  To place a mark, such as a signature or a stamp, on a document to signify that it is authentic, effective, or valid.  To approve or adopt a writing as one’s own.