filter
in order theory, a nonempty, downward‐directed, upward‐closed subset of a preordered set
neighborhood system
(for a point x) collection of all neighborhoods for the point x
generic filter
in set theory, given a collection of dense open subsets of a poset, a filter that meets all sets in that collection
club filter
the filter on a regular uncountable cardinal consisting of all club sets
filter
family of sets representing "large" sets