Ceviche (pronounced "seh-VEE-chay") is a Latin American recipe for raw fish and seafood marinated in citrus juice, mainly lime and lemon juice. The acid in the citrus juice denatures the proteins in the fish, causing it to become opaque and producing a firm texture.
What does "denature" mean? Proteins are made up of long chains of amino acids, and when exposed to acid, these chains break apart and arrange themselves into different configurations.
A more common way of denaturing proteins is by exposing them to heat (i.e. "cooking" them). So with ceviche, the fish is effectively "cooked" even though there's no heat involved.
As such, ceviche is served either chilled or at room temperature.
Note that while heat is an effective method for killing the bacteria that cause food poisoning, marinating it in citrus juice is not. So it's important to use the freshest seafood you can, from a source you trust.
Other ingredients in ceviche typically include onions, chiles and cilantro, and sometimes tomatoes. Avocado is a wonderful item to include in ceviche, and as a bonus, the acid will prevent the avocados from turning brown.
Ceviche recipes vary by region, and many different fish and shellfish can be used in preparing it. Snapper, sea bass, halibut, mah-mahi and tilapia are popular fish for making ceviche. Other seafood components can include shrimp, scallops, squid and octopus.
In Ecuador, shrimp ceviche is made solely with shrimp and includes tomato sauce. The Peruvian version can include corn and cooked sweet potato.
One will sometimes hear the word ceviche mispronounced by leaving off the third syllable, thus rendering it "seh-VEECH" instead of "seh-VEE-chay." The effect of this can be compared, not favorably, to fingernails on a chalkboard, especially because the most common offenders tend to be professional cooks, who ought to know better.
Probably the most important thing when making ceviche (other than how you pronounce it) is making sure to marinate it for the proper amount of time. Too short and your seafood will be raw. Too long and it'll be rubbery in the case of shrimp or octopus, or chalky and crumbly in the case of fish. Fifteen to 20 minutes is usually about right.
Incidentally, an interesting thing about ceviche is that it proves that marinating does not tenderize meat. People often claim that the acid in a marinade helps break down tough cuts of meat, causing them to become tender. But that's not what they do at all. As we see, when acid interacts with protein, it causes them to become more firm, not more tender.