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From Proto-Indo-European *h₂erk- (to protect, guard, hold, lock). Cognates include Latin arca (chest, box), arceō (I defend), arcānus (hidden, secret), arcera (kind of wagon), Old Armenian արգել (argel, obstacle) and Ancient Greek ἀρκέω (arkéō).



arx f (genitive arcis); third declension

  1. stronghold, castle, citadel, fortress, acropolis
    • Vergil, Aeneid, 2.56:
      ... Trōiaque, nunc stārēs, Priamīque arx alta, manērēs.
      ... and Troy, you would now be standing, and Priam's mighty citadel still endure.
    1. (figuratively) defence, protection, refuge, bulwark
    2. tyranny (with arx as the abode of tyrants)
  2. (metonymically) height, summit, pinnacle, top, peak (since castles were often built on heights)


Third-declension noun (i-stem).

Case Singular Plural
Nominative arx arcēs
Genitive arcis arcium
Dative arcī arcibus
Accusative arcem arcēs
Ablative arce arcibus
Vocative arx arcēs


  • arx in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
  • arx in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
  • arx in Charles du Fresne du Cange’s Glossarium Mediæ et Infimæ Latinitatis (augmented edition with additions by D. P. Carpenterius, Adelungius and others, edited by Léopold Favre, 1883–1887)
  • arx in Gaffiot, Félix (1934) Dictionnaire illustré Latin-Français, Hachette
  • arx in Harry Thurston Peck, editor (1898) Harper's Dictionary of Classical Antiquities, New York: Harper & Brothers
  • arx in Samuel Ball Platner (1929), Thomas Ashby, editor, A Topographical Dictionary of Ancient Rome, London: Oxford University Press
  • arx in William Smith et al., editor (1890) A Dictionary of Greek and Roman Antiquities, London: William Wayte. G. E. Marindin