An immutable characteristic is any sort of physical attribute which is perceived as being unchangeable, entrenched and innate. The term is often used to describe segments of the population which share such attributes and are contrasted from others by those attributes, and is used in human rights law to classify protected groups of people who should be protected from civil or criminal actions which are directed against those immutable characteristics.
A major debate over sexual orientation is over whether or not non-heterosexuality is a valid physical attribute, with religious and social conservatives often perceiving sexual orientation as a "correctable" behavior.