Mother-in-law: the mother of one's spouse (usage and nuance vary by region/context).
姑は週末によく孫の世話をしてくれる。
My mother-in-law often takes care of the grandchildren on weekends.
彼の姑は厳しい人で有名だ。
His mother-in-law is known to be strict.
日本では姑と嫁の関係が話題になることがある。
In Japan, the relationship between mother-in-law and daughter-in-law is sometimes a topic.