Nonbinary is an umbrella term used to describe gender identities outside of the gender binary of male and female. It is often used synonymously with genderqueer. The identity is generally considered to be under the transgender umbrella, but not all nonbinary people self-identify as transgender. People who identify as nonbinary may also feel that their gender is both male and female or no gender at all. These identities are known as bigender and agender respectively.
Being nonbinary is not a modern concept. Nonbinary people have existed for centuries, with even some religious figures in different cultures being described as neither male nor female. Similarly to other sects of the queer population, the concept of nonbinary gender expression has not recently appeared; instead it has lost the intense stigma associated with it that would prevent nonbinary people from coming out as such.