The most common choice is to take "small" to mean finite.
In this case, two sets are almost disjoint if their intersection is finite, i.e. if (Here, '|X |' denotes the cardinality of X, and '< ∞' means 'finite'.)
For example, the closed intervals [0, 1] and [1, 2] are almost disjoint, because their intersection is the finite set {1}.
However, the unit interval [0, 1] and the set of rational numbers Q are not almost disjoint, because their intersection is infinite.
of the natural numbers has been the object of intense study.