Transsex, also known as Transexual/Transsexual is a sex label to refer to someone whose current or desired sex characteristics doesn’t match their original natural sex characteristics. This may include, differing genitals, gonads, or other secondary sex characteristics. A transsex person either has changed some or all sex characteristics, or desires to change these characteristics, typically through hormone replacement therapy, or sex reassignment surgery. Transsex people may also be considered altersex.
Transsex is not the same as transgender. Transgender people who don’t plan to physically transition would not be considered transsex. Additionally, not all transsex people are transgender. It should also be noted that not all transgender people that wish to transition reclaim the term transsex/transexxual. The term should not be applied to someone unless they express that they are fine with it.
The opposite of transsex is cissex.
The term transsexual is a medicalized term that was coined in the 1923 in Germany when Dora Richter, the first known person to successfully complete male-to-female gender reassignment-surgery medically transitioned. Dora Richter began transition in 1921 and was under Magnus Hirschfeld's care until 1930. In 1923 Hirschfeld introduced the German term "transsexualismus." Afterwards David Oliver Cauldwell introduced "transsexualism" and "transsexual" to English in 1949 and 1950.
Cauldwell seems to be the first to utilize the term to refer to people who wish to medically transition. In 1969, Harry Benjamin was claimed to be the first person to use the term "transsexual" in a public lecture of which he gave in 1953. Benjamin went on give the term popularity with his book, The Transsexual Phenomenon in 1966.
Benjamin's book described transsexualism on a scale which was later called the Benjamin Scale. He described them in three scales: non-surgical, moderate intensity and high intensity. In Benjamin's book, he described "true" transsexualism as the following:
True transsexuals feel that they belong to the other sex, they want to be and function as members of the opposite sex, not only to appear as such. For them, their sex organs, the primary (testes) as well as the secondary (penis and others) are disgusting deformities that must be changed by the surgeon's knife.
In 1965 John Oliven coined the term "transgender." By the 1990's the term "transsexual" became a subset under the transgender label. The term transgender is now more common and a majority of the trans community prefer the term "transgender" over "transsexual."