The term "gender identity" refers to a person's
perception of their own gender.
The vast majority of adults are cisgender: their gender
identity matches their genetic (a.k.a. biological) gender. That is:
Persons with female genitalia, ovaries, and DNA containing XX chromosomes have a female genetic gender. They regard themselves as females.
Almost all males have male gentilia, XY chromosomes in their
DNA, and perceive themselves as male.
However, a very small minority of persons are transgender.
They typically have a genetic gender that is opposite of their gender identity.
They are sometimes described as having a female brain and a male body, or
Transgender persons are often considered to have gender dysphoria: emotional difficulty caused by the difference between their gender identity and biological gender. Most
therapists used the term Gender Identity Disorder (GID) prior to 2012. Since then, Gender Dysphoria has become the preferred term. Conservative
Christians often use the term "Gender Confusion Disorder" apparently to
imply that it is a figment of the person's imagination and easily cured via therapy.
The term "transsexual" is often used to refer to a transgender
person who has engaged in hormone treatments and, optionally sexual reassignment
surgery, so that their body more closely resembles their sexual identity. It is gradually falling out of use.