Final answer:
A signature on the reverse side of a check is called an endorsement, which signals the approval to transfer or cash the check. It must not be confused with terms like counterfeit, which refer to fraudulent imitations.
Step-by-step explanation:
The signature on the reverse side of a check is commonly known as an endorsement. This is not to be confused with a counterfeit, backer's mark, or authorization mark, which are terms not typically used in this context. When a person endorses a check, they are signifying their approval to either transfer the funds to another party or to cash the check themselves. Endorsements can be done in different ways, including a blank endorsement where only the signature is present, or a restrictive endorsement which includes specific instructions about how the check can be used.