An adult baby is a mature person who likes pretending to be an infant. An adult baby may play the role of a baby full-time or only some of the time. The lifestyle adult babies practice is known as infantilism.
Being an adult baby can be a way for some people to enjoy a sexual fetish known as paraphilic infantilism or adult baby syndrome. However, some adult babies have no sexual motivation for their role playing activities. Most adult babies play infants of their own gender. However, some adult babies, known as sissy babies, play infants of the opposite gender.