The Collaborative International Dictionary of English v.0.48:

Naturism \Na"tur*ism\, n. (Med.)
   The belief or doctrine that attributes everything to nature
   as a sanative agent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

naturism
    n 1: going without clothes as a social practice [syn: nudism,
         naturism]