Gratuitous exposure of a character's panties, possibly the biggest fan service concept in anime and manga.
A Panty Shot refers to when a person's panties are shown, usually unintentionally. This can happen in a number of ways, the most common of which include wind flipping a skirt or someone bending over, revealing their underwear. People who don't wear skirts or dresses aren't guaranteed safe haven from the dreaded Panty Shot either, as other types of clothes can, on occasion, slip or get pulled off.
The Panty Shot or Panchira as it's referred to in Japan started appearing in anime and manga as early as the 1950s or 60s and has become a staple of fan service-themed franchises. It's arguably the most recurring fan service concept in the entire medium. Characters are usually deeply embarrassed after their panties are seen by others, though some are completely oblivious to it.