also: しょたいけん
first experience; first time doing something
General use for any novel experience, not necessarily sexual. Often used with する to mean 'try something for the first time'.
Since it was my first experience traveling abroad, I was very nervous.
彼は初体験のスポーツにすぐ慣れた。
He quickly got used to the sport he was trying for the first time.
first sexual experience; losing one's virginity
Euphemistic or direct reference to one's first sexual encounter. Context usually makes the meaning clear.
彼女は高校生のときに初体験をした。
She had her first sexual experience when she was in high school.