Ichthys, sometimes spelled Ichthus, or Ikhthus, from Koine Greek: ΙΧΘΥΣ or ΙΧΘΥϹ is the ancient and classical Greek word for “fish.” In English it refers to a symbol consisting of two intersecting arcs, the ends of the right side extending beyond the meeting point so as to resemble the profile of a fish, used by Early Christians as a secret symbol and now known colloquially as the “sign of the fish” or the “Jesus Fish.”
ΙΧΘΥΣ (Ichthys) is an acronym for “Iēsous Christos, Theou Yios, Sōtēr, which translates into English as “Jesus Christ, God’s Son, Savior”. According to tradition, ancient Christians, during their persecution by the Roman Empire in the first few centuries after Christ, used the fish symbol to mark meeting places and tombs, or to distinguish friends from foes. (Wiki)