fish: the water-dwelling animal that people catch, cook, and eat
This is the ordinary word for a fish, the everyday creature of lake and sea. In the New Testament it turns up almost entirely in scenes from around the Sea of Galilee, where fishing was a common trade and fish were a dietary staple: the boy's five loaves and two fish that feed the crowd, the nets so full they begin to tear, the coin found in a fish's mouth, and the piece of broiled fish the risen Jesus eats to show he is not a ghost. Paul uses it once in a wider way, listing fish alongside people, animals, and birds as one of the distinct kinds of living flesh.
Greek has more than one word in this area, and they are not quite interchangeable. This word names the living creature itself. When the Gospels speak of fish already caught and prepared as something to eat with bread, they often switch to a different word (ὀψάριον), and there is also a diminutive for a small fish (ἰχθύδιον). The distinctions are gentle, but they explain why one Gospel scene can use two different "fish" words within a few verses.
Because the meaning is so plain and stable, the word carries no hidden theological weight of its own. Its interest lies almost entirely in the vivid, concrete settings where it appears.