What is "formation rule"

n. (context logic English) One of a set of rules ("'''formation rules'''") used to construct well-formed formulas of a formal language.

In mathematical logic, formation rules are rules for describing which strings of symbols formed from the alphabet of a formal language are syntactically valid within the language. These rules only address the location and manipulation of the strings of the language. It does not describe anything else about a language, such as its semantics (i.e. what the strings mean). (See also formal grammar).