A mob cap or mob-cap is a round, gathered, or pleated cloth (usually linen) hat consisting of a caul to cover the hair, a frilled or ruffled brim, and (often) a ribbon band to keep it in place (modern versions use elastic).
Worn as an indoors hat by married women in the 18th & early 19th century. Depending on the location and era, it was also worn by maids as an alternative to the stereotypical maid headdress. During the time period, it was called a bonnet but is not to be confused with the more general use of the term. Use this tag in preference to the other when applicable rather than as a sub-type.
Commonly associated with the Touhou games because of the author's fondness for putting them on many of his more popular, early Windows-era characters.