A fruit native to Latin America. It's pronounced in the Spanish fashion: the double "l" sounds like an English "y", so "tom-ah-TEE-yo". It's also known as the Mexican green tomato, jamberry, tomate, tomate verde, or fresadilla. Like its close cousin the tomato, the tomatillo is a member of the nightshade family.
Tomatillos look like small green tomatoes except that they're covered by a thin parchment-like covering. Unlike green tomatoes, which are very sharp in flavour, tomatillos have a pleasant lemony tang which lends a bright flavour to Mexican dishes. Though they can become yellow, they are generally picked when underripe and sold when green; buy them at Hispanic markets, and look for fruits which are firm and have dry, tight-fitting husks.
Tomatillos can be used raw in salads and sauces, though be aware that they are quite acidic. Slice them thinly and sprinkle them on salad, or grind them up in a food processor to make salsa fresca. However, they are more usually cooked because this enhances their flavour and softens their thick skins. The easiest way to do so is probably to remove the husks, wash off the sticky coating, cut off the stems, cover them with cold water, and simmer till they are soft but not bursting open, about 5 to 15 minutes, depending on their size. At this point they can be used to make sauce or salsa, added to guacamole, or frozen for future use. Most delicious, however, is roasting or grilling on a hot dry griddle, under the broiler, or on a barbecue until their skin is charred and the flesh is softened; cool and use to make sauce or salsa. In any case, I recommend adding onion, garlic, hot peppers, and coriander to your tomatillo concoction.
Fresh tomatillos will keep for up to a month, unwashed, in a paper bag. You can also buy tomatillos canned, but the taste isn't as good: they taste, well, like a tin, and they're also quite mushy. If you can't buy them fresh, this may be your only option, but first check out your friendly neighbourhood Latin American market, where you may well find the original item, if only sporadically. (You'll find the canned ones there too.)