Definition from Wiktionary, the free dictionary
- A relative by marriage (or through affinity). Affinal relative; affine.
- Sometimes specifically a parent-in-law of one's child, for which the dedicated terms co-mother-in-law, co-father-in-law are rare
relative by marriage
parent-in-law of one's child
father-in-law — see father-in-law
mother-in-law — see mother-in-law