Define a thermoset.

A thermoset is best defined as a type of plastic material that, once cured through a chemical reaction, cannot be remolded or reheated without undergoing decomposition. This process usually involves a cross-linking of the polymer chains, which creates a rigid structure that provides strength and heat resistance. Because of this permanent setting, thermosets are ideal for applications requiring durability and thermal stability, such as in electrical insulators and automotive parts.

In contrast, the other options describe properties of different types of plastics. Reusable or thermoplastic materials, for example, can be reheated and reshaped multiple times, making them quite versatile in manufacturing processes. Decomposition upon reheating characterizes thermosets explicitly. Flexible plastics typically refer to elastomers or other variants that can be bent or stretched, which does not accurately describe the rigidity typical of thermosets. Thus, acknowledging the irreversible nature of thermosets in terms of their chemical structure after curing is essential to understanding their applications and limitations in manufacturing processes.