Today several types of apron designs are popularly referred to as pinafore aprons.
In a narrow sense it is a full apron with two holes for the arms that is tied or buttoned in the back, usually below the neck. While this is a garment for women of all ages, it was commonly worn by girls in America and Western Europe during the late 19th and early 20th.
The term pinafore apron is also often applied to a frilly bib apron with shoulder straps that criss cross in the back and tie in a ribbon. Occasionally standard bib aprons are referred to as pinafore aprons, but this usage is confusing and should be avoided.