maternal aunt

Short definition from Wiktionary

Noun

1. 1
   An aunt on one's mother's side of the family, including both: A sister of one's mother.
2. 2
   An aunt on one's mother's side of the family, including both: A sister-in-law of one's mother; the wife or ex-wife of a maternal uncle.
3. 3
   A similar great aunt, etc.