Deprecating the "X-" Prefix and Similar Constructs in Application Protocols

 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".

...

http://tools.ietf.org/html/rfc6648