331

331 is a natural number that comes after 330 and before 332. In mathematics, 331 is a prime number. It is the 67th prime and is congruent to 3 modulo 4. As a prime of the form 4k + 3, it cannot be expressed as a sum of two squares of integers. In base 2, 331 is written as 101001011.

In history, the year 331 BCE is notable for Alexander the Great's decisive victory at the Battle of Gaugamela, which led to the rapid downfall of the Achaemenid Empire and expanded his dominion into Asia. The year 331 CE lies in late antiquity and saw ongoing political and cultural developments across the Roman world.

In the North American Numbering Plan, 331 is an area code assigned to parts of northern Illinois, including communities in the Chicago metropolitan area.

In computing, the FTP protocol uses the status code 331 to indicate that the username was accepted and a password is required to proceed.

In popular culture, 331 appears in product names such as the Nokia 3310 mobile phone, released in 2000.