The true definition of a fantasy burrito varies from person to person; such is the nature of fantasy. However, the generally accepted definition (and who could ask for anything more?) is thus: A fantasy burrito is an event, point in time, or object, in which a person has multiple fantasies realized at the same time or during the same event.
1. A man sitting at a bar with a redhead on the right and an asian on the left, is enjoying his redhead/asian/bar fantasy burrito.
2. A man enjoys baked potatoes, anchovies, and steak and dreams of eating them all at once. If at any point he puts them all into a burrito and eats it, he too would be enjoying his (quite literal) fantasy burrito.