Asian basil is an umbrella term for the aromatic basil varieties used across cuisines in Southeast Asia, most often Thai basil and holy basil. The two names are frequently used as if interchangeable, but they refer to botanically distinct plants: Thai basil is a variety of the same species as common sweet basil (Ocimum basilicum), while holy basil, also called tulsi, is a separate species (Ocimum tenuiflorum) with a spicier, clove-like flavor. Substituting one for the other changes the dish.
Holy basil in particular has roles beyond the kitchen: in India it is central to traditional Ayurvedic medicine and Hindu ritual, and in Thailand it appears in Buddhist ceremonies. Its leaves have long been credited with medicinal properties in these traditions.
Both herbs are staple ingredients in Thai, Vietnamese, and Lao cuisines. Thai basil, the more commonly used of the two outside specific signature dishes, has a flavor profile built on sweetness, licorice-like anise notes, and a subtle peppery undertone, and pairs well with meats, seafood, vegetables, and rice dishes. Holy basil is spicier and more pungent, and is used specifically where that character is wanted.
When used in cooking, Asian basil is usually added toward the end of cooking to preserve its delicate flavor and aroma. It appears in stir-fries, curries, soups, and salads. In Thai cuisine, holy basil is the defining ingredient in Pad Kra Pao (stir-fried minced meat with holy basil), while Thai basil is more typical in dishes such as Tom Kha Gai (coconut milk soup with chicken and galangal).
Both herbs are also used in herbal teas and infusions, and their growing popularity in the West means they are increasingly available fresh, though they are still often substituted for one another or for sweet basil when unavailable, at a real cost to a dish's intended flavor.
Related but separate varieties sometimes marketed under the same 'Asian basil' label include the purple-hued Purple Basil and lemon-scented Lemon Basil, both true varieties of Ocimum basilicum rather than holy basil.
Overall, 'Asian basil' is a convenient but imprecise umbrella label: knowing which of the two plants a recipe actually calls for matters more, in practice, than the umbrella term itself.