The common letter-sized paper used in Europe and in most locations outside of North America. At 8.3 x 11.7" (210x297mm), A4 paper is slightly narrower and a little longer than "A" size 8.5 x 11" paper (216x279mm).
The A Series
The A4 is part of the ISO "A Series," where A0, the first dimension, equals almost one square meter (841x1189mm = 999,949 square mm). Starting with A1, each size is one half the square surface area of the previous size while maintaining the same aspect ratio. For more sizes, see engineering drawing sizes
Most printers default to scaling incoming documents between A4 and A paper sizes so they can be printed on the prevailing paper stock. If "paper substitution" is not set, the printer will attempt to ensure that the paper size matches the document size.