TeX category code (\catcode)

A TeX category code tells the TeX input processor how to treat an input character syntactically. Categories include escape 0, beginning-group 1, ending-group 2, letter 11, other 12 and active 13. The usual backslash and brace conventions are assignments of these categories, rather than immutable properties of the printed characters.^[ref]

For example, suppose a initially has category 11. Defining \def\demo{a} stores that character token. The assignment \catcode 97=12 changes how a subsequently read character with code 97 is classified. It does not change the earlier character token inside \demo. The example assumes the ordinary character-code mapping for a and leaves the letters of demo unchanged.

The distinction matters for reusable TeX macros: a definition can preserve syntax that would be read differently under later category assignments. It also explains how changing letter categories can alter the scanning of a TeX control word without changing the visible spelling of the source.