WordNet (r) 3.0 (2006):

fuzzy logic
    n 1: a form of mathematical logic in which truth can assume a
         continuum of values between 0 and 1

The Free On-line Dictionary of Computing (30 December 2018):

fuzzy logic
fuzzy computing

   A superset of Boolean logic dealing with the concept of
   partial truth -- truth values between "completely true" and
   "completely false".  It was introduced by Dr. Lotfi Zadeh of
   UCB in the 1960's as a means to model the uncertainty of
   natural language.

   Any specific theory may be generalised from a discrete (or
   "crisp") form to a continuous (fuzzy) form, e.g. "fuzzy
   calculus", "fuzzy differential equations" etc.  Fuzzy logic
   replaces Boolean truth values with degrees of truth which are
   very similar to probabilities except that they need not sum to
   one.  Instead of an assertion pred(X), meaning that X
   definitely has the property associated with predicate
   "pred", we have a truth function truth(pred(X)) which gives
   the degree of truth that X has that property.  We can combine
   such values using the standard definitions of fuzzy logic:

   	truth(not x)   = 1.0 - truth(x)
   	truth(x and y) = minimum (truth(x), truth(y))
   	truth(x or y)  = maximum (truth(x), truth(y))

   (There are other possible definitions for "and" and "or",
   e.g. using sum and product).  If truth values are restricted to
   0 and 1 then these functions behave just like their Boolean
   counterparts.  This is known as the "extension principle".

   Just as a Boolean predicate asserts that its argument
   definitely belongs to some subset of all objects, a fuzzy
   predicate gives the degree of truth with which its argument
   belongs to a fuzzy subset.

   Usenet newsgroup: news:comp.ai.fuzzy.

   E-mail servers: ,
   , .

   (ftp://ftp.hiof.no/pub/Fuzzy),
   (ftp://ntia.its.bldrdoc.gov/pub/fuzzy).

   FAQ
   (ftp://rtfm.mit.edu/pub/usenet-by-group/comp.answers/fuzzy-logic).

   James Brule, "Fuzzy systems - a tutorial", 1985
   (http://life.anu.edu.au/complex_systems/fuzzy.html).

   STB Software Catalog
   (http://krakatoa.jsc.nasa.gov/stb/catalog.html), includes a
   few fuzzy tools.

   [H.J. Zimmerman, "Fuzzy Sets, Decision Making and Expert
   Systems", Kluwer, Dordrecht, 1987].

   ["Fuzzy Logic, State of the Art", Ed. R. Lowen, Marc Roubens,
   Theory and Decision Library, D: System theory, Knowledge
   Engineering and Problem Solving 12, Kluwer, Dordrecht, 1993,
   ISBN 0-7923-2324-6].

   (1995-02-21)