ich
noun
variants:
also ick \ ˈik 
\
\
plural ichs
also icks
Medical Definition of ich
: a severe dermatitis of freshwater fish caused by a protozoan of the genus Ichthyophthirius (I. multifiliis) and especially destructive in aquariums and hatcheries
— called also ichthyophthiriasis, ichthyophthirius