The word Ichthys (ΙΧΘΥΣ) is actually an acronym, made up of the following:
- Iota (i) is the first letter of Iēsous (Ἰησοῦς), Greek for "Jesus".
- Chi (ch) is the first letter of Christos (Χριστός), Greek for "anointed".
- Theta (th) is the first letter of Theou (Θεοῦ), Greek for "God's", the genitive case of Θεóς, Theos", Greek for "God".
- Upsilon (y) is the first letter of (h)yios (Ὑἱός), Greek for "Son".
- Sigma (s) is the first letter of sōtēr (Σωτήρ), Greek for "Saviour".
The symbol its self consists of two intersecting arcs which resemble the profile of a fish. The symbol was adopted by early Christians as a secret symbol. It is now known also as the "sign of the fish" or the "Jesus fish".
Greeks, Romans, and many other pagans used the fish symbol before Christians. Hence the fish, unlike, say, the cross, attracted little suspicion, making it a perfect secret symbol for persecuted believers.
The fish has plenty of other theological overtones as well, for Christ fed the 5,000 with 2 fishes and 5 loaves and called his disciples "fishers of men."
See the Vesica Piscis Symbol for the possible, believed by some, origin of this symbol.