In accounting and finance, an endorsement is the act of signing a negotiable instrument (like a check) to transfer it to another party. When you sign the back of a check, you are endorsing it — authorizing the bank to process it.
There are different types: a blank endorsement (just a signature), a restrictive endorsement ("For deposit only"), and a special endorsement (transferring to a specific person).
Beyond checks, endorsement also means formal approval or support. A manager endorsing a purchase order means they approve the spending.