A reserved character is a single letter or symbol that cannot be used because it's utilized by the operating system or programming language for specific functionality. For example, many operating systems reserve the following characters and disallow them from being used when saving or renaming a file.
\, /, :, *, ?, ", <, >, and |
These characters are reserved because they're used with operating system functions, such as the slashes separating folders in a directory path and * being a wildcard character.
With programming languages and markup languages, reserved characters depend on the language. For example, the less than ( < ) and greater than ( > ) symbols are reserved characters because they're part of HTML tags.
Reserved characters can either not be used at all, need to be escaped, or require a special code.
Character, Escape sequence, Programming terms, Reserved word