Ceviche, seviche, or cebiche - your choice - is almost a cuisine in its own right. Enormously popular in the western countries of South America as well as Mexico, the Caribbean, and other parts of Latin America, it has many variations, but is basically a simple blending of fish and citrus juice, with the addition of vegetables and spices.
The chemical process that occurs when the acid of the citrus comes in contact with the fish is similar what happens when the fish is cooked, and the flesh becomes opaque and firm. Indeed, many people refer to the juice as "cooking" the fish, although that is just plain wrong!
South American lemons are not as sweet as those in this country, and are often used for the ceviche. Further north, limes are the fruit of choice, although many people use a mix of lemon and lime.
Ceviche can include a mixture of saltwater fish, scallops or other shellfish, squid or octopus, and onion, chili, tomato, pepper and/or cilantro, and is often served as an appetizer or light meal. In this country, red snapper, sole, and pompano are the most popular choices. We have some fans of scallop ceviche in the office here, although shellfish is usually cooked briefly first (blanched) and oiled before going into the lime juice, as the acid tends to break down its texture if it is raw.
Many ceviche recipes ask you to marinate the fish for a good four hours to set the fish all the way through, although some call for much shorter marinating times. Keeping the fish in the juice longer (say, overnight) results in a lime taste that many people think overpowers the fish.
The one cardinal rule of preparing ceviche is that the fish must be absolutely fresh. There are quite a few ceviche recipes available online.