According to Liz and James Egger, authors of the "Complete Guide to Nudism and Naturism" (2006), nudists are "normal, morally responsible respectable citizens - singles, couples and families - who just happen to share a passion for wholesome social nudity."
The book is a great introductory resource to the modern situation of nudism. I'm sure I'll refer to the book again.
The authors also provide a concise definition of naturism/nudism:
"Naturism is the enjoyment of getting as naked as possible, wherever appropriate, alone or with others, just for its own sake"
It's a nice definition. Clear and to the point.
The book is available here.