The mother-in-law is the mother of a spouse. They are usually a member of the wedding party.
If a couple has yet to marry, the mother of a future spouse is often referred to on message boards as the FMIL (future mother-in law).
If a parent has re-married, there can be multiple mother-in-laws. The step-mother of the spouse would technically be the step-mother-in-law. Some spouses may choose not to make this distinction.