Definition of fish louse in US English:

fish louse

noun

  • An aquatic crustacean which is a parasite of fish, typically attached to the skin or gills.

    an elongated crustacean that becomes permanently attached to the host and typically highly modified (class Copepoda: several orders and numerous species)

    a free-swimming crustacean with a shieldlike carapace and a pair of suckers (class Branchiura: several genera, in particular Argulus)