Someone who ignores judgemental/uninformed arguments like "waaah but we are part of the food-chain, its natural to eat animal products." argument. This argument is pretty much identical to any person when asked or threatened with any behavior out of the ordinary that needs to use kindergarden logic to defend themselves. When a non-vegan hears the word vegan they generally have a look of disgust on their face and are more annoying than most vegans themselves with all of their patronizing questions/arguments they think are witty.
Some vegans are self-absorbed and moralistic a-holes, often the ones from PETA are the worst of these. Some vegans don't push their personal beliefs on others and get annoyed with how the preachy ones ruin everything for them.
non-vegan: Want some eggs?
vegan: no thanks.
non-vegan: OH MY GAWWWD WTF DO YOU EAT? WHY? Isn't it...like natural to eat animal products? You need calcium and shit!
vegan:meh, its working for me, I like how I am now, thanks.
non-vegan: WTF YOU ARE SOOO PREACHY!