A type of logic using real numbers between 0 and 1 instead of True and False as values. Attempt to encapsulate the "shades of grey" notions of truth of natural language. The compatibility of Lojban (in at least some part based on classical logic) and fuzzy logic is a recurring topic of discussion.
The usual operator for expressing fuzzy logical truth value in Lojban is "jei" (CLL chapter 11.6). A subscript to "ja'a" may also be used (CLL chapter 19.6).
Related dmoz category is http://www.ifi.unizh.ch/ailab/aiwiki/aiw.cgi?CategoryFuzzyLogic