  Let us first introduce the formal notion of definability as follows. If L is a language, A an L-structure, and X a subset of the domain A of A, we say that X is definable in A by a formula φ(x, y1,…,yn) of L if there is a sequence a1,…,an of elements of A such that X is the subset of all elements x ∈ A for which φ(x, a1,…,an) holds in A.