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). The name "mob cap" comes from its common fashion among rioting, poorer women during the French revolution, as it was popularly worn as an indoors hat by married women in the 18th & early 19th century.
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.