401

401 is an integer in the decimal system. In mathematics it is a prime number, meaning it has no positive divisors other than 1 and itself.

In computing, 401 is the HTTP status code for Unauthorized. It indicates that a request requires user authentication and that the provided credentials are missing, invalid, or insufficient. It is distinct from 403 Forbidden, which indicates that access is understood but denied.

401(k) refers to a common type of defined contribution retirement plan in the United States. It is named after a provision of the Internal Revenue Code. Employees may contribute pre-tax dollars, and many employers offer matching contributions. The funds typically grow tax-deferred until withdrawal in retirement.

Area code 401 covers most of Rhode Island and is the state's sole NANP area code.

Ontario Highway 401, often simply called the 401, is a major east–west freeway across Southern Ontario. It runs from Windsor in the west to the Ontario–Quebec border in the east, and passes through major cities such as London and Toronto, handling substantial traffic.

Historically, 401 can denote the year 401 CE in the Gregorian calendar, a point in late antiquity used in historical chronology.