Noun
subformula (plural subformulas or subformulae)
(logic) A formula that is part of another formula.
Eventually the formula should be reduced to its atoms, but if some subformula cannot be so reduced, then the formula is not a wff. Source: Internet
The set of free variables in a formula of L κω can have any cardinality strictly less than κ, yet only finitely many of them can be in the scope of any quantifier when a formula appears as a subformula of another. Source: Internet