"Naturism" is considered to be the same as "nudism" by most people in the US and Canada. Some regions and countries tend to use "naturism" more than the word "nudism". The nature part of the word reflects the "harmony with nature" aspect of being nude.
The MSN Encarta definition of naturism is this:
wearing no clothes as custom: the practice of going without clothes, usually in a communal setting or in designated areas, in the belief that nudity is a healthy natural state
The Encarta definition mentions the social setting aspects of naturism. However, solitary nudist hiking qualifies as naturism just as much as riding a bike in the nude with nudists for a cause.