definendum

Definendum is a rarely used variant of the term defined in logic and philosophy. In standard terminology, the element that is being defined is called the definIendum, and the defining expression is called the definiens. The form definiendum is widely adopted, while definendum itself is considered nonstandard or a misspelling in many contemporary sources.

In a definitional pair, the definIendum is the word or phrase whose meaning is being specified, and the definiens is the phrase that provides the definition. For example, in the statement “A square is a rectangle with equal sides,” the definiIendum is square, and the definiens is “a rectangle with equal sides.” If one encounters definendum, it is generally interpreted as an alternative spelling or mistaken usage of definiendum rather than as a distinct technical term.

Etymology and usage notes: The terms come from Latin, with definire meaning to define and the suffix -ndum indicating “that which is to be.” The standard modern form is definiendum; definendum appears primarily as a typographical or historical variant. In formal discussions of definitions, clarity is achieved by using the conventional trio: definiendum, definiens, and the definition itself.

See also: definiendum, definiens, definition, logical definition, lexical definition.