A law in (2-valued) logic which states there is no third alternative to truth or falsehood. In other words, for any statement , either or not- must be true and the other must be false. This law no longer holds in three-valued logic or fuzzy logic.

Erickson, G. W. and Fossa, J. A. Dictionary of Paradox. Lanham, MD: University Press of America, pp. 64-65, 1998.

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Weisstein, Eric W. "Law of the Excluded Middle." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/LawoftheExcludedMiddle.html