What term describes something that is held back or limited?

The term that describes something that is held back or limited is "retard." This word comes from the Latin root "retardare," which means to slow down or cause to move more slowly. In various contexts, including scientific or technical discussions, it is used to refer to processes or actions that are hindered or restrained. In contrast, the other options imply an increase or improvement: "advance" suggests progression, "boost" indicates an increase or enhancement, and "enhance" means to improve or augment something. Therefore, "retard" accurately captures the idea of limitation or restriction.