Illformed

Illformed, or ill-formed, is an adjective used to describe expressions that do not conform to a prescribed grammar, syntax, or structure. The term is used across multiple disciplines to distinguish nonconforming strings from well-formed ones. In practice, illformedness is defined relative to a formal or conventional set of rules defining what counts as valid.

In linguistics, an ill-formed sentence is one that violates the syntax, morphology, or agreement rules of a language. Ill-formedness can result from incorrect word order, missing auxiliary elements, or improper agreement. Some researchers distinguish ill-formed strings from semantically odd but syntactically acceptable strings; others treat semantic unacceptable sentences as ill-formed.

In formal languages and logic, ill-formed strings are those not belonging to the language generated by a grammar, while well-formed formulas satisfy the formation rules. Examples include “P and” or “(P) Q” which do not follow the grammar for the language. The distinction is central to parsing and proof systems.

In computing, ill-formed data or input fails to satisfy the constraints of a data format or protocol, such as ill-formed XML lacking closing tags or proper nesting, or an ill-formed program that does not compile due to syntax errors. Many systems provide error handling to reject ill-formed input and provide diagnostics.

Because ill-formedness depends on the governing grammar, what is ill-formed in one context may be acceptable in another; the term is relative rather than absolute.