Lewis & Short

Ăchĕron, ntis (collat. form Acheros, Liv. 8, 24, 11; the form Acheruns, untis, see below), m., = Ἀχέρων (interpr. ὁ ἄχεα ῤέων, the stream of woe).

  1. I. A river in Epirus, which flows through the Lake Acherusia into the Ambracian Gulf, now Suli, Liv. 8, 24, 3; Plin. 4, 1, 1, § 4.
  2. II. A fabulous river in the Lower World: illi qui fluere apud inferos dicuntur, Acheron, Cocytus, Styx, etc., Cic. N. D. 3, 17: via Tartarei quae fert Acherontis ad undas, Verg. A., 6, 295 al.
    Hence,
    1. B. The Lower World itself: Acherontem obibo, ubi mortis thesauri objacent, Enn. ap. Fest. p. 201 Müll. (Trag. v. 278 ed. Vahl.): flectere si nequeo Superos, Acheronta movebo, Verg. A. 7, 312: perrupit Acheronta Herculeus labor, Hor. C. 1, 3, 36. In prose: ut eum suo sanguine ab Acheronte, si possent, cuperent redimere, Nep. Dion. 10, 2.
      Hence, Ăchĕrontēus, a, um, adj., pertaining to the Acheron, Claud. Rapt. Pros. 2, 351.