What term is used for non-denominational first-class postage?

The term "Forever Stamps" refers to non-denominational first-class postage issued by the USPS. These stamps can be used to mail a letter regardless of future price increases for first-class postage. This means that a Forever Stamp will always be valid for sending a standard letter, even if the rate changes after the stamp's purchase. They are designed to offer consumers an easy and worry-free way to send mail without having to calculate the current postage rate at the time of mailing.

In contrast, other types of stamps listed do not provide this flexibility. Regular stamps typically denote stamps with a specific postage value, international stamps are designated for international mail with differing rates, and Priority Mail stamps are for expedited services rather than standard first-class mail. Thus, the unique feature of Forever Stamps being non-denominational and usable indefinitely for first-class mail makes it the correct term in this context.