relaxer

A relaxer is something or someone that relaxes.

Most commonly it refers to a chemical treatment applied to tightly curled or coiled hair to straighten it permanently by breaking down the hair's protein structure.

She gets a relaxer every few months to maintain her straight look.

The term also applies more generally to anything that promotes relaxation, such as a muscle relaxer (a medication that reduces muscle tension) or simply a calming activity or substance.