A well-formed formula is said to be true for the interpretation (written ) iff every sequence in (the set of all denumerable sequences of elements of the domain of ), satisfies . is said to be false for iff no sequence in satisfies .

Then an interpretation is said to be a model for a set of well-formed formulas iff every well-formed formula in is true for (Mendelson 1997, pp. 59-60).