In cooking, mayonnaise is a thick, white, creamy, cold sauce; an emulsion of fat (originally olive oil, now usually vegetable oil) suspended in a matrix of egg yolks, flavored with vinegar or lemon juice, and seasonings.

Mayonnaise is made by slowly introducing oil into the other liquids while whisking vigorously to break up the fat into small droplets that will become dispersed in the liquid. The egg yolks provide lecithin, which stabilizes the emulsion. It is then seasoned with salt and sometimes other seasonings.

Mayonnaise is only eaten cold, for instance on sandwiches (mainly in North America) and on French fries (mainly in northern Europe). It is also often used as a base for many other cold sauces with more varied ingredients. Aioli, for example, is a garlic-flavored mayonnaise. Popular herbed mayonnaises include tartar sauce. Many salad dressings are mayonnaise-based, including "Russian" dressing, which is a combination of mayonnaise with tomato sauce, and "Thousand Island", which is Russian dressing with pickles and herbs.

Homemade mayonnaise can approach 75% fat before the emulsion breaks down; commercial mayonnaises are more typically 65-70% fat. Commercial products typically replace much or all of the egg yolk with water, requiring the addition of lecithin or other emulsifiers from sources such as soy (some commercial mayonnaises may thus be appropriate for vegans). "Low fat" mayonnaise products contain starches, cellulose gel, or other ingredients to simulate the texture of real mayonnaise.

Since homemade mayonnaise contains raw egg yolks, it poses a danger of salmonella poisoning. Commercial producers pasteurize the yolks, or freeze them, and substitute water for most of their liquid, or use other emulsifiers. At home, be sure to use the freshest eggs possible, and thoroughly clean them before use. Some stores sell pasteurized eggs for home use. You can also coddle the eggs in 170° water and remove the hot yolks from the whites, which will have cooked slightly. Homemade mayonnaise will only keep under refrigeration for three to four days. A lower-fat version can be made with "silken" tofu.

Commercial mayonnaise sold in jars was born on Manhattan's Upper West Side. In 1905, the first ready-made mayonnaise was sold at Richard Hellman's delicatessen on Columbus Avenue, between 83rd and 84th Streets. In 1912, Mrs Hellman's mayonnaise was mass marketed and called "Hellman's Blue Ribbon Mayonnaise."

The origin of sauce Mayonnaise.

Mayonnaise made its English-language debut in a cookbook of 1841. Mayonnaise is generally said to have been created by the chef of the Duc de Richelieu in 1756, to celebrate the Duc's victory over the British at the port of Mahon (the capital of Minorca in the Balearic Islands). It is supposedly from that port's name that the word mayonnaise is derived. But this often-repeated story seems flawed.

Antoine Careme speculated in 1833 that the name was derived from the French word manier, meaning 'to handle, to feel, to ply,' thus possibly in this case 'to stir or blend'. Careme appears to have been straining to come up with an etymology for sauce 'Mayonnaise' . It is inconceivable that Careme, trained by the greatest patissier in Napoleonic Paris, and chef d'hotel to the duc de Talleyrand, with whom he spent an hour each morning working out the day's menus, at whose table Careme virtually created French haute cuisine, should have been unaware of the fact, if mayonnaise had actually been created as recently as 1756. Indeed, Talleyrand himself had already held a bishopric under the Ancien regime, was a fastidious connoisseur of the table and moved in much the same circles as the Richelieu family. The origin of 'mayonnaise' must be much older than 1756, if it was obscure to Careme.

In fact it may appear more credible that sauce Mayonnaise was originally named for Charles of Lorraine, Duke of Mayenne (in northwest France), who presided over the meeting of the Estates General in January 1593 that had been summoned for the purpose of choosing a Catholic ruler for France. The sauce may have remained unnamed until after the Battle of Arques in 1589. It may then have been christened “Mayennaise” in 'honor' of Charles de Lorraine, duc de Mayenne, because he took the time to finish his meal of chicken with cold sauce before being defeated in battle by Henri IV.

It should be noted that the duc de Mayenne's close ally at the head of the Catholic party in France during the Wars of Religion, was the Queen Mother, Catherine de' Medici, who is generally credited with introducing into Parisian cooking olive oil, which was as traditional in the queen's native Italy as butter and lard were in the Ile-de-France.