Icthus is the English transliteration for the Greek word for 'fish'. In English, it refers to the Christian symbol of a fish which identifies one as a Christian. See the image in the Related Link below.
Boogie Baby: If you want the christian meaning for Icthus, it came from The words meaning
Iesous
Chreistos
Theou
Uios
Soter (Jesus Christ God's Son, Savior)
Together in the word Icthus it translates to fish, hence the little Jesus Fish, during times of the persecution of Christians, it was used as a secret symbol to identify other Christians without being caught, and the whole time the Roman king thought they were talking about the pagan god! :)