A vegan is an individual who has chosen a diet that does not contain any animal products. In addition to not eating meat, vegans also do not consume dairy products or eggs.
As Vitamin B-12 is generally is not found in plant sources, vegans usually have to take supplements. However, most other nutritional requirements can be met easily with such a diet as long as a good mix of foods are chosen.
In Resignation, Honey, the nutritionist who brings in her boyfriend to the clinic and later agrees to go out with House is a strict vegan and breaks up with her boyfriend when he "cheats" and eats a hamburger.
Also see Vegetarian