Lewis & Short

ăvuncŭlus (AVOMCVLVS and AVONCLVS, Fabr. Gloss. p. 227, a), i, m. dim. [avus; cf. Lith. avynas, uncle], a mother’s brother, maternal uncle (a brother of the father, patruus).

  1. I. Lit.
    1. A. In gen., Cic. de Or. 2, 84, 341; Verg. A. 3, 343; Plin. 5, 8, 6, § 20; Vulg. Gen. 28, 2; 29, 10: uxor patrui vel avunculi, ib. Lev. 20, 10; Isid. Orig. 9, 6, 17; cf. Dig. 38, 10, 10 P.
      Hence,
    2. B. Esp.
      1. 1. Avunculus magnus, a grandmother’s brother (aviae frater), great-uncle, Cic. Brut. 62, 222; Dig. 38, 10, 1; 38, 10, 10; Isid. Orig. 9, 16, 26.
      2. 2. Avunculus major, a brother of the great-grandmother, greatgreat-uncle (proaviae frater), Dig. l. l. (in Isid. Orig. l. l., proavunculus).
      3. 3. Avunculus maximus, a brother of the great-greatgrandmother (abaviae frater), Dig. l. l. (in Isid. Orig. l. l., abavunculus).
        In the histt. sometimes avunculus major = avunculus magnus, brother of the grandmother, Vell. 2, 59; Suet. Aug. 7; id. Claud. 3; and avunculus absol. = avunculus major, Tac. A. 2, 43; 2, 53; 4, 75.
  2. II. Transf., the husband of the mother’s sister, Sen. Cons. ad Helv. 17.