The term "mother-in-law" refers to the mother of one's spouse. However, there are several synonyms that can be used to refer to this important part of the family. Some common synonyms for "mother-in-law" include mother of one's spouse, spouse's mother, mother of one's partner, and the matriarch of the family. Other terms include "mom-in-law," "second mom," "bonus mom," and "mother by marriage." These terms reflect a range of relationships and attitudes towards one's mother-in-law, with some emphasizing a strong bond and closeness while others maintain a more formal distance.