Fermat's Theorem
There are so many theorems due to Fermat that the term "Fermat's theorem" is best avoided unless augmented by a description of which theorem of Fermat is under discussion.
Fermat's little theorem is sometimes known as Fermat's theorem (Hardy and Wright 1979, p. 63).
See also
Fermat's 4n+1 Theorem, Fermat's Last Theorem, Fermat's Little TheoremExplore with Wolfram|Alpha
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References
Hardy, G. H. and Wright, E. M. An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Clarendon Press, pp. 13 and 219, 1979.Referenced on Wolfram|Alpha
Fermat's TheoremCite this as:
Weisstein, Eric W. "Fermat's Theorem." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/FermatsTheorem.html