Veganism is the ideas and underlying principles and beliefs as to why a person inevitably adopts a vegan diet. 'Vegan' only defines the type of diet.
Veganism has a commonality in principles, but is not exclusive, nor limited to the following;
Ethical - enabling animals to have rights to prevent torture, rape and murder of all sentient beings.
Environmental Sustainability - The impact animal production has on world. Ref. United Nations Report '07: Livestock’s long shadow.
Health - Global Famine, The shortening of life spans, and obesity ~ mainly heart related illnesses. Animal born Diseases, Athleticism.
People adopt some, the whole or no beliefs, yet are still vegans. The main point is that whatever path we have taken; it was a strong enough conviction to adopt a vegan lifestyle.
Q: How could you be a vegan, yet not believe in veganism?
A: Simple, by adopting the diet, yet rejecting the principles.
A person who tries to wear a "I'm saving the animals" badge, with going to little to no effort, by eating fish instead of our 4-legged friends (GM Chicken).
They are considered annoying, as they have no moral grounding, hence the play with words, "Pesky"+"tarian".
'Yeah I don't like to eat animals, only fish as they have no feelings and have short memories'
'Oh I c, ur a Peskitarian...'