The primary ingredient in mayonnaise is egg yolk. Mayonnaise is an emulsion consisting mainly of egg yolk, oil, vinegar or lemon juice, and seasonings. The egg yolk acts as a stabilizing agent, allowing the oil and water (from vinegar or lemon juice) to blend smoothly together instead of separating. The emulsifying properties of lecithin, found in egg yolk, play a critical role in achieving the thick and creamy texture that mayonnaise is known for. Different variations of mayonnaise exist, with additional flavors and ingredients, but the base typically revolves around the emulsification driven by the egg yolk.