The KEYWORD package contains symbols, called keywords, that are typically used as special markers in programs and their associated data expressions.
Symbol tokens that start with a package marker are parsed by the Lisp reader as symbols in the KEYWORD package; see Section 2.3.4 (Symbols as Tokens). This makes it notationally convenient to use keywords when communicating between programs in different packages. For example, the mechanism for passing keyword parameters in a call uses keywords to name the corresponding arguments; see Section 3.4.1 (Ordinary Lambda Lists).
Symbols in the KEYWORD package are, by definition, of type keyword.
18.104.22.168.1 Interning a Symbol in the KEYWORD Package
22.214.171.124.2 Notes about The KEYWORD Package