This word is used for a person who lives as, and understands themselves to be, a different sex from the one they were said to be at birth — often (though not always) someone who has had medical treatment such as hormones or surgery. It works as a noun ('she is a transsexual') and as an adjective ('a transsexual woman'). The important thing for a student is not the definition but the feeling of the word. It is an old medical term, and today most trans people in English-speaking countries do not use it about themselves; many find it cold or even rude, because it puts all the weight on surgery and on sex. The normal, safe, polite word now is simply 'transgender', or 'trans' — 'a trans woman', 'a trans man'. Some older people still say 'transsexual' with no bad meaning at all, and a few people do choose it for themselves; you will also meet it in books and papers written before about 2010, where it was the ordinary word. So: understand it when you read it, and choose 'trans' or 'transgender' when you speak. One more thing worth knowing: 'a trans woman' means a woman who was called a boy at birth — the word before 'woman' tells you her history, not what she is now.