One’s internal sense of being male, female, neither of these, both, or other gender(s). This is separate from the sex one was assigned at birth, and can be different from one’s gender expression.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

The individual identification of a person's gender, as defined by that person. Usually used in reference to transgender, genderqueer, or transsexual people -- people whose gender identities differ from their physical sex.

The schizophrenic belief that you are anything but a man, woman, or hermaphrodite; despite being a man or woman on a biological and physical level.

Usually proclaimed by 3rd-wave feminists in order to get more attention or make themselves feel special. Be especially careful of saying "sir" or "ma'am" in California, New York, or Illinois. You might get arrested or fined.

A persons unchanging identity based on the gender of their brain.

Usually this will match a persons sex but in some cases when the brain doesn't get enough hormones at birth the brain can develop to be opposite to a person's sex or be genderless and the person will be born transgender.