Reserved:

   Many URL schemes reserve certain characters for a special meaning:
   their appearance in the scheme-specific part of the URL has a
   designated semantics. If the character corresponding to an octet is
   reserved in a scheme, the octet must be encoded.  The characters ";",
   "/", "?", ":", "@", "=" and "&" are the characters which may be
   reserved for special meaning within a scheme. No other characters may
   be reserved within a scheme.