Mayonnaise is a thick, creamy white condiment made by emulsifying oil, egg yolks, and some form of acid—usually vinegar or lemon juice.
It's sold commerically in jars, but you can easily make your own. Commercial mayonnaise is also available in reduced-fat and fat-free versions which rely on heavily on modified food starch and other emulsifiers to attain the appropriate texture and consistency.
Homemade mayonnaise takes a little more time to make, but with the advent of electric mixers and blenders, it's simple and the results are well worth it.
Mayonnaise is a classic spread for sandwiches, but it also provides a base for other sauces and dressings like aioli, remoulade, and thousand island.
Commercial mayonnaise will keep in the refrigerator for about six months, homemade mayonnaise, on the other hand, is only good for a few days.