Greek word meaning "fish", pronounced ichthus.
Why is the fish a symbol for Christians? Why do these fish sometimes have the Greek word for fish inside them?
I know this much, when you ask me to answer questions about Christianity, you're making a Devil's wager. But this I can say for sure:
It's an acrostic:
Ι -- Ιησους "Jesus"
Χ -- Χριστος "Christ"
Θ -- Θεου "God's"
Υ -- Υιος "Son,"
Σ -- Σωτηρ "Saviour"
Nifty, huh. Now go out among the nations of men.