Which term describes a method that impacts the structure of hair permanently?

The term that describes a method that impacts the structure of hair permanently is chemical change. This refers to a process in which the chemical composition of the hair is altered, resulting in lasting modifications to its structure. Chemical changes can occur during treatments such as perms or relaxers, which fundamentally reconfigure the hair's bonds, leading to a new texture or shape that remains until the hair grows out or is cut.

While chemical relaxing entails a specific process aimed at straightening curly hair by breaking the disulfide bonds, it's a subset of the broader category of chemical changes. Similarly, chemical properties refer to the characteristics of substances that become apparent during a chemical reaction, and base direction is a term related to how curls or waves are directed during styling, but it does not encompass the concept of permanently altering hair structure. Therefore, the most accurate term for a method that permanently changes the hair's structure is chemical change.