Mile High Club
The Mile High Club is a colloquial term for people who have engaged in sexual activity aboard an airplane during a flight. It does not refer to an actual club membership, but to a popular erotic cliché. A minimum altitude of one nautical mile is not a requirement; that detail is simply part of the expression.
In practice, sex on an airplane is usually impractical, cramped, and associated with legal and hygiene issues. It can also disturb other passengers and is prohibited in many situations. It should therefore not be attempted lightly; if intimacy while traveling is desired, private and legally unproblematic settings are the better choice.