Tomatillo is the ingredient that gives salsa verde its characteristic green color in Mexican cuisine.
A tomatillo is the small, green fruit of Physalis philadelphica, a nightshade related to tomatoes, eggplants and peppers. It grows inside a papery husk and has a naturally tart, bright flavor. That combination of color and acidity makes it especially useful as the base of many Mexican green sauces.
For salsa verde, tomatillos may be used raw, boiled or roasted, then blended with ingredients such as chile, onion, garlic and cilantro. Cooking changes their texture and flavor, but the tomatillo remains the defining green fruit. Green chiles contribute color and heat, yet they are not the canonical answer to this question.
The name can cause confusion because tomatillos are sometimes called Mexican husk tomatoes, but they are not simply unripe standard tomatoes. Mature tomatillos can also turn yellow, purple or reddish, so not every tomatillo is green; the green variety is the familiar salsa-verde ingredient.