They are a specific style of hosiery. Unlike full pantyhose, this style ends just about mid-thigh, instead of going all the way up to your waist. Exactly where a pair of thigh highs ends on your leg will depend on your height and inseam. There are two basic thigh high styles: stay-ups the type that stays up on their own , and traditional the type that doesn't.
This is a great article outlining the details of vintage fashion! Great job! Many modern women do not know the difference between the different hosiery, so today I'm doing a special little lesson in Lycra education.