maternal grandmother
Formal or written term for one's maternal grandmother. In everyday speech, おばあちゃん or 祖母 (そぼ) are more common.
外祖母 is a word that refers to one's maternal grandmother.
Compound of 外 (がい, 'outside; external'), referring to the maternal side in traditional family structure, and 祖母 (そぼ, 'grandmother').