A woman wearing a skirt reveals, inadvertently or purposefully to a person physically below them, that they are not wearing panties, the new information, and the inevitable glance at her pussy resulting in humiliation for either or both parties, usually depending on if the woman upskirted on purpose(voyeur embarrassed), the voyeur looked up her skirt(woman embarrassed), or a completely accidental upskirt(both parties embarrassed, often depending on the confidence and comfortability of the characters.)