Definitions for fuzzy logic
Random House Webster's College Dictionary
the theory of a mathematical set (fuzz′y set′) having the property that each of its members is described in terms of a number, with a value in the range from 0 to 1, that indicates the degree on a spectrum of values to which the member belongs to the set.
Origin of fuzzy logic:
a form of mathematical logic in which truth can assume a continuum of values between 0 and 1
A form of reasoning, derived from fuzzy set theory, whereby a truth value need not be exactly zero (false) or one (true), but rather can be zero, one, or any value in between.
U.S. National Library of Medicine
Approximate, quantitative reasoning that is concerned with the linguistic ambiguity which exists in natural or synthetic language. At its core are variables such as good, bad, and young as well as modifiers such as more, less, and very. These ordinary terms represent fuzzy sets in a particular problem. Fuzzy logic plays a key role in many medical expert systems.