Medical Definition of ich
plural ichs also icks
: 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
Seen and Heard
What made you want to look up ich? Please tell us where you read or heard it (including the quote, if possible).