Variable APR

A variable APR is an interest rate that moves with an index, usually the prime rate: the card or loan adds a fixed margin to the index, so the APR rises and falls with it.

Almost all credit cards have variable APRs. The margin (for example, prime + 18.74%) is set when you open the account; the index changes. Your agreement says how soon changes apply, usually from the next billing cycle.

A rise in the prime rate isn't an issuer "raising your rate" under the CARD Act, which limits discretionary increases on existing balances.

Sources: CFPB: Regulation Z, 12 CFR 1026.55