Something written on the back of a document.
An alternate spelling, in some English jurisdictions, is "indorsement."
In the laws of bills of exchange, an endorsement is a signature on the back of the bill of exchange by which the person to whom the note is payable transfers it by thus making the note payable to the bearer or to a specific person.
An endorsement of claim means that if you want to ask a court to issue a writ against someone, you have to "endorse" your writ with a concise summary of the facts supporting the claim, sometimes called a statement of claim.
When used in the context of insurance law, an endorsement is a written amendment as to the coverage of an existing property or liability insurance policy. An amendment or modification of a life or health insurance policy is referred to as a rider.