Mayonnaise is an emulsion of egg yolks and oil with vinegar and/or mustard. There are many theories as to where the name came from however the chef Marie Antonin is generally credited with creating the original recipe in 1828.
If you prefer your mayonnaise thinner, add a little cold water. Mayonnaise can be varied by adding spices, herbs or fresh ingredients. For inspiration, see Adam Stoke's Rosemary mayonnaise, Galton Blackiston’s Mango mayonnaise, Liz Allen’s Curry mayonnaise, Vineet Bhatia’s Beetroot mayonnaise or Nathan Outlaw's Lime mayonnaise.