Someone who chooses to not eat anything animal.
A vegan who puts sufficient effort in to learning how to eat a healthy vegan diet is not unhealthy and skeletal looking, like some vegans who lack knowledge of what constitutes a healthy vegan diet.
A vegan puts effort in to their food choices that goes beyond "convenience food" and is healthier than the Standard American Diet--which gives them a good chance of not dieing from one of the major causes of old-age related death in the Western world--heart disease--and perhaps even cancer, depending on the diet.
A vegan learns that cows milk or other animal's milk are not the only sources of calcium. Protein and other things thought to only come from animals, can also come from vegetarian sources. In fact, with a little culinary knowledge it is easy to eat a wide variety of tasty vegetarian dishes--India's been doing it for thousands of years!
A vegan may know that it is not only protein that accounts for muscle growth, any knowledgeable body-builder can attest to that. Nutrients such as Zinc, b-vitamins, and a generally healthy diet are absolutely critical to muscle growth. Something meat is generally high in, but can be had from a vegan diet.
Avi R. Lehyani, a vegan body builder, who resides in Israel and is on the website veganbodybuilding dot com, is proof that vegans can be strong--contrary to some peoples ignorance. Who can bench press 380lbs and can rack lift 836lbs.
Guy who has experience with one vegan before learning about veganism: Frank just become a vegan, he's been raving about it for the last few months to everyone he knows! ... judging and praising people intensely. He looks very skinny, he's obviously not eating right. He probably needs some meat in his diet or something to get his calcium... there's 'no way' he can get enough of those in his diet!
Guy after seeing the people from veganbodybuilding dot com and having been a vegan for several years:
I had no idea vegans could be so strong until I saw Avi Lehyani, these guys know how to be strong AND vegan, what a seeming contradiction! When I first went vegan I had no idea how to be anything over 120lbs, then i started eating right and I gained over 10lbs a month, what a difference healthy eating makes! It IS possible to be strong and be vegan!