Aioli is a French sauce made with garlic, egg, and olive oil. If you describe aioli as "fancy mayonnaise," you're not wrong!
Although aioli is extremely similar to mayonnaise, there are a few basic differences. While mayo is usually an emulsion of canola oil, egg yolk, vinegar or lemon juice, and salt, garlic is at the heart of aioli. Traditionally, it's made by crushing a lot of garlic and thoroughly mixing it with olive oil (and sometimes egg). Purists insist that this is true aioli, a word that comes from the French ail, "garlic," and the Latin oleum, "oil."