An aquatic crustacean which is a parasite of fish, typically attached to the skin or gills:
- ● A free-swimming crustacean with a shield-like carapace and a pair of suckers (class Branchiura: several genera, in particular Argulus). ● An elongated crustacean that becomes permanently attached to the host and typically highly modified (class Copepoda: several orders and numerous species).
Definition of fish louse in:
- The US English dictionary