Anisometropia is when your two eyes have significantly different prescriptions, one more short-sighted or long-sighted than the other. Antimetropia is a specific version of that, where one eye is short-sighted and the other is long-sighted, opposite directions entirely. Both make standard lens fitting harder, since your brain has to reconcile two very different images, so we approach the lens design differently for each eye rather than treating the prescription as one combined number.