Counterexample

From CS2800 wiki

To disprove that a predicate [math]P [/math] holds for all [math]x [/math], you only need to choose a specific [math]x [/math] (called a counterexample) and show that [math]P [/math] is false. Put another way, the logical negation of "for all [math]x [/math], [math]P [/math]" is "there exists an [math]x [/math] such that [math]P [/math] is false".