ichthyarchynonce-wd.|ˈɪkθɪɑːkɪ| [f. ichthyo- + Gr. -αρχία rule, government.] The domain of fishes; the fish-world in all its orders. 1853Badham Halieut. (1854) 275 To back an Agnano..or Thrasymene ‘tenca’ against the whole of the Mediterranean ichthyarchy.