Transsexualism is the condition in which a person with an apparently normal somatic sexual differentiation has the conviction that he or she is actually a member of the opposite sex. This conviction which is accompanied by a profound sense of loathing for individuals's own primary and secondary sexual characteristics, is absolute, overwhelming and unalterable. The sense of belonging to a particular sex, not only biologically but also psychologically and socially, is called gender identity.

Money et al. (1955) coined the term gender identity and defined it as:

 The sameness, unity and persistence of one's individuality as male, female, or ambivalent, in greater or lesser degree, especially as it is experienced in self-awareness and behavior; gender identity is the private experience of gender role, and gender role is the public expression of gender identity, while gender role is defined as everything that a person says and does, to indicate to others or to the self the degree that one is either male, or female, or ambivalent; it includes but is not restricted to sexual arousal and response.

            Transsexualism can be defined as an incongruence between the biological sexual differentiation and the gender identity. For clinical and diagnostic purposes the criteria of the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association (3rd edition) are often used (DSM III, 1980 (4)).

          A. Sense of discomfort and inappropriateness about one's anatomic sex.

          B. Wish to be rid of one's own genitals and to live as a member of the other sex.

          C. The disturbance has been continuous (not limited to periods of stress) for at least two years.

          D. Absence of physical intersex or genetic abnormality.

          E. Not due to a coexistent mental disorder, such as schizophrenia.