<logic> A wff that is taken to be true without proof in the construction of a theory or stipulated as unproved premise for the proof of other wffs inside a formal system (comparelemma).
axiom schema (plural: schemata)
A formula containing variables of the metalanguage which becomes an axiom when its variables are instantiated to wffs of the formal language.
An axiom that is a logically valid wff of the language of the system. See logical validity
An axiom that is not a logically valid wff of the language of the system (but is a closed wff).
[Glossary of First-Order Logic] and [FOLDOC]
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