Mayonnaise is a thick, pale, creamy cold sauce made by whisking egg yolk together with oil and an acid such as lemon juice or vinegar until they form a smooth, glossy emulsion. Rich and unctuous on the tongue with a faint, bright tang, it is one of the great mother sauces of the cold kitchen and the base from which aioli, tartare, remoulade and a host of dressings are built.