Foolscap is paper cut to the size of 8 1⁄2 × 13 1⁄2 inches (216 × 343 mm), sometimes 13 × 8 in (330 × 200 mm). This was a traditional paper size used in Europe and the British Commonwealth, before the adoption of the international standard A4 paper, the most common standard size in the world. Foolscap was so called because, in the 18th century, folio-sized paper had the watermark of a fool's cap on it. Basically, it is taller than A4.
Paper of the size is now very rarely used, being superseded by A4 (297 x 210). However, some ring binders, lever arch files, suspended files, and other filing related things still use the size.