  The language  L(∞,ω)  may also be counted as a finite-quantifier language. The concept of equivalence of structures with respect to this language is of especial significance: we call two (similar) structures A and B (∞,ω)-equivalent, written A ≡∞ω B, if the same sentences of  L(∞,ω)  hold in both A and B. This relation can, first of all, be characterized in terms of the notion of partial isomorphism. A partial isomorphism between A and B is a nonempty family P of maps such that: