Lewis & Short

mātertĕra, ae, f. [for materitera, mater and an obsol. fem. noun found in neutr. i-terum; cf. al-tera, a second mother],

  1. I. a mother’s sister, an aunt by the mother’s side: amita est patris soror: matertera est matris soror, Dig. 38, 10, 10, § 4; cf. ib. § 5: mea matertera (anulum gestitavit), Plaut. Curc. 5, 2, 4; Cic. Div. 1, 46, 104; id. de Or. 2, 1, 2; Ov. M. 3, 312: materteram uxorem ducere non licet, Gai. Inst. 1, 62; Mos. et Rom. Leg. Coll. 6, 2, 2.
  2. II. Esp.
    1. A. Magna, i. e. aviae soror, Dig. 38, 10, 1, § 15; cf.: matertera patris et matris, mihi magna matertera est, Paul. ex Fest. p. 136 Müll.
    2. B. Major, i. e. soror proaviae, Dig. 38, 10, 1, § 16.
    3. C. Maxima, i. e. abaviae soror, Dig. 38, 10, 1, § 17.