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, eggs, or honey.
As Vitamin B12 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