404 is an HTTP status code. When you visit a web page, your computer is requesting data from a server using Hypertext Transfer Protocol (HTTP).

Before the web page is displayed, your browser sends the web server its HTTP header. The server responds with its HTTP header, which contains the status code.

For a normal web page, the status is 200 (OK). You don't see this because the server just sends you the content of the page.

If you encounter an error, then you see the actual status code, such as 404 Not Found.

The first 4 indicates a client error. The server is saying that you've done something wrong, such as misspell the URL or request a page which is no longer there. (For comparison, a 5xx error indicates a server-side problem, which may be transient; if you try it again, it may work.)

The middle 0 refers to a general syntax error. This could indicate a spelling mistake.

The last 4 just indicates the specific error in the 40x group, which also includes 400: Bad Request, 401: Unauthorized, etc.
404 not found
by johnhms July 12, 2008
not found
sorry, this page cannot be found
by alex lowbrow March 09, 2004
(v.i.) Out to lunch; not having it together; acting in a stupid way. Typically derogatory.

Derivation: the "404: Not Found" error encountered when surfing the web.
That luser's 404!

Today in class, Hsu was 404.
by The Grammar Nazi March 19, 2001
A highway in Toronto, Ontario, Canada, which, ironically is home to HP, Intel, Xerox and IBM offices.
I totalled my Chevy on the 404.
by IanD August 24, 2005
Adjective: a vacant, stupid person. From the result given when you look for a website that is not there.
That guy's 404--he barely knows his own name.
by Stu April 13, 2003
To be 404'ed is to be given an invalid phone number, email address, etc. by someone you want to contact again after a pleasurable, but brief, encounter, as in a crowded bar with your friends crowding in on your action.
"Man, I hit on this fine chick last night at the club. I asked for her number, but when I called this morning I realized she 404'ed me!"
by Burro1949 June 02, 2006
A phrase used when one expects money to be available to them, only to realise that this resource is not actually available at the time it is needed.
Mandem 1 - oi oi, you still on goin' halves yeah?
Mandem 2 - on it, one sec..... Oh shit, you gotta be rampin'?! 404. Money not found!
Mandem 1 - Par.
by mdzc July 27, 2011

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