[¦lek·sə·kō¦graf·ik ′ȯr·dər] (mathematics) Given sets A and B with a common ordering ,>A and of elements of B by (a1, a2,…) <>b1, b2,…) if either ai= bifor every i, or an<>bn, where n is the first place in which they differ; this is the way words are ordered in a dictionary.