The term "A3" refers to a specific size for paper. An A3 sheet of paper measures 297 x 420 millimeters (11.69 x 16.54 inches.) This size is standardized by the ISO paper size system.
The International Organization for Standardization, or ISO, provides "A" series standardized paper measurements. The A series paper sizes range from A0 (largest) to A10 (smallest.) Due to the specific height-to-width ratio used by the ISO system, any size page folded in half along its shortest side is equal to the next smallest page measurements. For example, an A3 page folded in half along it's shortest side will have the same measurements as an A4 page.
The ISO paper size system is rarely used in the American market. The American alternative to an A3 page would be tabloid or ledger size, which measure 279 x 432 millimeters or 11 x 17 inches.