What makes a sauce qualify as mayonnaise?

To be called mayonnaise, a sauce must meet strict criteria. Not all sauces labelled mayonnaise are technically mayonnaise. Today, the European industry has agreed that mayonnaise must be an oil-in-water emulsion containing at least 70% fat and at least 5% egg yolk. Products like lighter mayonnaises with less fat do not meet these standards and cannot be marketed as mayonnaise.