A4 paper is a standard international paper size defined by ISO 216, measuring 210 x 297 millimeters (8.27 x 11.69 inches), commonly used globally for office printing, notes, and documents, offering a generous writing area similar to US Letter size but slightly narrower. It's part of the A-series, where each subsequent size (A5, A6, etc.) is half the area of the previous one, making it versatile for everything from flyers to large posters.