This is the noun for the condition or state of being transsexual — that is, of being a person who lives as a different sex from the one given at birth, often with medical help. Where 'transsexual' names the person, 'transsexualism' names the whole thing as a topic: 'a study of transsexualism', 'his research on transsexualism'. That tells you where you will meet it: in medical writing, psychology books, and law, mostly from the twentieth century. Nobody says it in ordinary conversation. Like 'transsexual', it now sounds old and clinical, and many trans people dislike it, partly because the ending -ism makes it sound like an illness or a strange belief, the way 'alcoholism' does. Modern writing says 'being transgender', or talks about 'gender identity' or 'gender dysphoria' (the medical name for the distress some trans people feel). So treat this as a reading word: know it when you see it in an older text, and do not reach for it yourself.

跨性別狀態／變性現象（舊醫學術語，現在幾乎只出現在舊文獻，日常不用）