In the context of credit cards, an authorized user generally means any person granted express, implied or apparent authority to use the credit card or credit card account of a cardholder. The term includes a secondary cardholder. A person doesn't necessarily need to be named on a credit card contract to be an authorized user.
When a person is an authorized user, he or she is authorized to use the card, but is not always responsible for paying. Nonetheless, FICO still scores authorized user accounts. This means that the primary account holder's credit rating can be penalized for an account that that holder is not responsible for paying. It's worse when there is a divorce, and you are still tagged as an authorized user on an ex-spouse's account that is unpaid and sent to collection. Precise definitions vary by credit card contract and state laws.