Ceviche ( seh-VEESH ), Raw fish and/or shellfish in a citrus marinade.a South American appetizer in which fresh raw fish is marinated in lime juice to "cook" it, along with onions, peppers and tomatoes. Tried it for the first time in Mexico and ever since have loved it. Its very easy to put together.Almost like making your typical Salsa, except you use shrimp or other frirm fish.