Authorized user
Someone a credit cardholder adds to their account so they can use the card. An authorized user generally doesn't owe the bill.
The person who opened the account, the primary cardholder, can add an authorized user. The user may get a card in their name, but the primary cardholder signed the agreement and is responsible for paying the balance.
Many card companies report the account on the authorized user's credit report too. A card with on-time payments and a low balance can help the user build credit. Late payments or a high balance can hurt it.
Example
Ana's dad adds her to his 15-year-old card. The card's on-time history can show on her credit report, even though she never uses it.