The little death
In the late 1800s and early 1900s la petite mort simply meant the brief loss (or near loss) of consciousness. A fainting spell, or severe dizziness, as one might experience after a nasty shock.
In 1949 Arthur Koestler used it to refer to the "self-transcending component" of an orgasm; apparently referring to the loss of sense of self one might feel when orgasming. It is often used in this sense, to link the orgasm to death.
It has more recently gained the meaning of passing out (or at least blacking out) during an orgasm. (This phenomenon is more common in females than males, although uncommon in both). This may have originated in the 1970s, perhaps in The Joy of Sex.
It is sometimes written La petite morte, but I believe that this actually translates to 'little dead girl', and is incorrect.