Mayonnaise is a thick, creamy sauce or dressing that is made of oil, egg yolks, lemon juice or vinegar, and seasonings. It's not the same as salad dressing, which doesn't contain egg yolks and is generally sweeter than mayonnaise.
History of Mayo
Mayonnaise was invented in 1756 by the French chef of the Duc de Richelieu. After the Duc beat the British at Port Mahon, his chef created a victory feast that was to include a sauce made of cream and eggs. Realizing that there was no cream in the kitchen, the chef substituted olive oil for the cream and a new culinary creation was born. The chef named the new sauce "Mahonnaise" in honor of the Duc's victory.