True aioli is an emulsion of garlic, olive oil, and salt. Over time things like egg, mustard, and lemon have been added to help with the emulsification process. While similar to a mayo in that there is egg emulsified with oil to make a creamy sauce, they are still different. Aioli is traditionally made with olive oil, while mayonnaise is made with a lighter more neutral oil, giving them distinctly different flavors.