What does the X- prefix mean in HTTP? (e.g. X-API-KEY)

Question

Consider this example from Wikipedia:

E.g.

GET /something HTTP/1.1
X-API-Key: abcdef12345

What does the X- technically stand for?

X-prefix just shows that this header is a custom one. You may have a look at this question: https://stackoverflow.com/questions/3561381/custom-http-headers-naming-conventions — Random Guy, Jun 06 '20 at 17:33

score 7 · Accepted Answer · edited Oct 07 '21 at 10:55

From RFC-6648, "X" is commonly understood to stand for "eXperimental" or "eXtension" :

Many application protocols use parameters with textual (as opposed to numerical) names to identify data (media types, header fields in Internet mail messages and HTTP requests, vCard parameters and properties, etc.). Historically, designers and implementers of application protocols have often distinguished between standardized and unstandardized parameters by prefixing the names of unstandardized parameters with the string "X-" or similar constructs (e.g., "x."), where the "X" is commonly understood to stand for"eXperimental" or "eXtension".

score 4 · Answer 2 · answered Jun 06 '20 at 17:33

4

The X is mostly used to indicate that is an unoffical, not officially specifed or application specific header.

answered Jun 06 '20 at 17:33

monty

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What does the X- prefix mean in HTTP? (e.g. X-API-KEY)

2 Answers2