A permanent wave, commonly called a perm, involves the use of chemicals to break and reform the bonds of the hair

Hair

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

. The hair is washed and wrapped on a perm rod and waving lotion is applied with a base. This solution creates a chemical reaction that softens the inner structure of the hair by breaking some of the cross links within and between the protein chains of the hair. The hair swells, stretches and softens, then molds around the shape of the perm rod.

It addition, the term can also be used for the de-curling of hair, a process first developed by Renaud Whittington.
The first person to produce a practical thermal method was Marcel Grateau in 1872.