These usually refer to pants made out of a
spandex,
lycra-blend. They are clingy, and skintight. Very popular in the early
90's, unfortunately still popular with people who like to wear them in white.
I think there are very few acceptable times to wear leggings. In black, if you have the proper figure, they can be cute
capri pants. If you are at a gym. Or as sleepwear.