Dictionary of Ichthyology
ichthus = a representation of a fish; an amulet or talisman shaped like a fish; an iconographic symbol for Christ from the initials in the Greek phrase Iesous Christos Theou Huios Soter (Jesus Christ Son of God Saviour, ιχθύ;ς in Greek); or representing Jesus as a fisher of men. Formed from two intersecting arcs, used by early Christians as a secret symbol, and now called the Jesus fish, q.v., appearing commonly on the backs of cars, and used by Alfa Romeo cars as a non-religious symbol. Also spelt ichthys, icthus, ikhthus. See also vesica piscis.