The key ingredient in pesto is basil. This aromatic herb provides pesto with its distinctive flavor and vibrant green color. Traditionally, pesto is made by blending fresh basil leaves with other ingredients, including garlic, pine nuts, Parmesan cheese, and olive oil, resulting in a delicious sauce that is commonly used with pasta, sandwiches, and salads. The fresh, peppery taste of basil is what sets pesto apart, making it a favorite in Italian cuisine. Variations of pesto can include other herbs or nuts, but basil remains the primary herb that defines this popular culinary staple.