In-laws

Does anyone know a term to describe the relationship
between in-laws, ie. what is the term to describe what the
parents of a son are in relationship to his mother-in-law
and his father-in-law?
Does anyone know of a source where one can post meanings
and get definitions?

An in-law is defined as any relative by marriage. I suppose that would include the parents of one's child's spouse-if our kinship system considers them one's relatives at all. I'm not sure it does. I don't know of a specific term for that relationship. The Western kinship system is rather stingy with terms, compared with some other systems.