of a set $X$
A subset $A$ of $X$ for which the relative complement $X \setminus A$ is finite.
The cofinite subsets of $X$, together with the empty set, constitute the open sets of a topology on $X$, known as the cofinite topology.
Cofinite subset. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cofinite_subset&oldid=39485