What is the primary characteristic of an endothermic wave in hairstyling?

The primary characteristic of an endothermic wave in hairstyling is that it is a perm that is activated by an outside heat source. In this context, endothermic waves rely on heat from an external source, such as a hooded dryer or a heat lamp, to initiate the chemical reaction that transforms the hair's structure. This type of perm is effective because the heat helps to open up the hair cuticle, allowing the chemicals to penetrate more effectively and alter the hair's shape.

In contrast, other types of perms might work solely through chemical reactions at room temperature or through natural heating methods in the case of certain products that do not require additional heat. This distinction makes endothermic waves unique since they specifically depend on that additional energy source to achieve desired results, enhancing their effectiveness.