What is the primary ingredient in traditional pesto sauce?

The primary ingredient in traditional pesto sauce is fresh basil. Pesto, a classic sauce originating from Italy, specifically the Liguria region, is characterized by its vibrant green color and aromatic flavor, which largely comes from the basil leaves. The essence of pesto lies in the combination of fresh basil with other ingredients like garlic, nuts, cheese, and olive oil, but basil is the key element that defines the sauce. Without basil, the distinctive flavor and identity of pesto would be significantly altered, showcasing its importance in this culinary staple.