- ichthyolThe ammonium salt of sulfonated bitumen; used to treat some forms of skin diseases
- ichthyophthiriasisA parasitic infection of freshwater fish caused by the ciliate Ichthyophthirius multifiliis.
- ichthyopterygiumfish fin.
- ichthyosaurAny fishlike marine reptiles of the extinct order †Ichthyosauria of the early Triassic to the late Cretaceous period, that had a body somewhat like a porpoise.