Entry 1 sense Webster, 1913 Ichthin /(ĭk"thĭn)/ · Ich·thin · IPA 01 n. A nitrogenous substance resembling vitellin, present in the egg yolk of cartilaginous fishes. 1. A nitrogenous substance resembling vitellin, present in the egg yolk of cartilaginous fishes.(Physiol. Chem.)