A logical structure which does not assume the law of the excluded middle. Three truth values are possible: true, false, or undecided. There are 3072 such logics.
Three-Valued Logic
See also
Fuzzy Logic, Law of the Excluded Middle, LogicExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Three-Valued Logic." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Three-ValuedLogic.html