「義母」（ぎぼ） is the woman who is married to your father but who is not your real mother, or the mother of your husband or wife.
In speech, 「義理の母」（ぎりの はは） may be used more often.
Likewise, 「義父（ぎふ）」「義兄（ぎけい）」「義弟（ぎてい）」「義姉（ぎし）」「義妹（ぎまい）」 are used.
「姑」（しゅうと） is also used for your mother-in-law.
