The American Heritage Stedman's Medical Dictionary, in its 2002 publication, provided the following definitions:
"1. A person with the external genitalia and secondary sexual characteristics of one sex, but whose personal identification and psychosocial configuration is that of the opposite sex.
2. A person who has undergone a sex change."
"1. Having both male and female characteristics, including in varying degrees reproductive organs, secondary sexual characteristics, and sexual behavior, as a result of an abnormality of the sex chromosomes or a hormonal imbalance during embryogenesis."
"1. Dressing or masquerading in the clothes of the opposite sex."
Eric Vilain, MD, PhD, stated in his Apr. 19, 2004 Los Angeles Times commentary titled "Gender Blender - Intersexual? Transsexual? Male, Female Aren't So Easy to Define":
"In reality, sex isn't so straightforward. Let's take testicles as a defining characteristic of a man. Are individuals with only one testis 'real' men? The 'two testicles rule' would disqualify about 3% of male newborns a year -- about 4.5 million Americans total. ... Adding a fertility criterion would eliminate millions more...
Since 1921, we have known that women have two X chromosomes and men an X and a Y chromosome...
Any genetics expert knows that there are exceptions to the chromosome rules. There are females with a Y chromosome; there are males with no SRY gene. At the Sydney Olympics in 2000, the IOC decided to 'refrain from performing gender tests,' conceding that no single test provided a complete answer.
Identifying the gender of intersex and transsexual individuals poses an even more complex challenge. Intersexuality is defined as the presence of 'ambiguous genital,' making it impossible to tell easily whether the newborn baby is a boy or a girl. It occurs at a frequency of 1 in 4,000 births...
Transsexuals believe that they have been born in the wrong body and often pursue a difficult and painful process of surgical reassignment. But the courts often don't recognize the change of sex and invalidate spousal rights of transsexuals...
Sex should be easily definable, but it's not. Our gender identity -- our profound sense of being male or female -- is independent from our anatomy."