Conull set
In measure theory, a conull set is a set whose complement is null, i.e., the measure of the complement is zero.<ref>Template:Citation.</ref> For example, the set of irrational numbers is a conull subset of the real line with Lebesgue measure.<ref>A related but slightly more complex example is given by Führ, p. 143.</ref>
A property that is true of the elements of a conull set is said to be true almost everywhere.<ref>Template:Citation. See p. 62 for an example of this usage.</ref>