mother-in-law: the mother of a woman's husband, the senior woman of the household that woman has married into
The word turns up only eleven times, and nine of them fall in the book of Ruth, where it works almost as Naomi's title. It never stands on its own: every time, it is somebody's mother-in-law, because the word names a position in a family rather than a person.
Hebrew keeps the two sides of a marriage apart with two different families of words. This word and its male partner, father-in-law (חָם), always point to the husband's parents. The wife's parents are named with a different set of words (חֹתֵן), which is why the law in Deuteronomy 27:23 about a man and his wife's mother does not use this word at all.
That one-sidedness matters because of how households worked. A married woman left her father's house and joined her husband's father's household, so her husband's mother was not a distant relative by marriage but the senior woman she now lived with and under. Ruth's story presses on exactly that point: both husbands are dead, the tie that had placed Ruth in Naomi's household died with them, and Ruth stays anyway. Boaz singles this out for praise in Ruth 2:11.
Micah 7:6 turns the same bond the other way. When a daughter-in-law (כַּלָּה) rises against her mother-in-law, the household has broken from the inside, and that is the prophet's picture of a whole society coming apart. Jesus quotes the line in Matthew 10:35 and Luke 12:53.