When a character who is officially and typically depicted wearing a hat is drawn without one while in a situation where you would expect him/her to be wearing a hat.
This is based on the character's costume and therefore doesn't apply to characters being nude or wearing a swimsuit.
Often applied to Touhou-related posts simply because of the abundance of hats within the series and the relative rarity of its characters being depicted without it.
If the hat is apparent in the picture but unworn, tag as hat removed instead.