(Ikhthus) The classical Greek word for 'fish', which some feel was used as a secret symbol for Christianity, along with a picture of a fish, when they were being persecuted.
ichthys, is actually five greek letters: iota, chi, theta, upsilon, and sigma. It's an acronym for "Iesus Christos theos, uios, sotor". This translates as "Jesus Christ, God's son, savior". But ichthys is also the Greek word for "fish".
The most famous secret sign in early Christianity was the Ichtys sometimes spelled ikhthus that resembles a fish and is today known as the Jesus fish. Early Christians developed this sign to mark meeting places or friend from foe. They had to do this because Early Christians were persecuted constantly by the Roman Empire up until it was made the official religion of the roman empire.