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Meaning of ultrafilter

ul•tra•fil•ter

Pronunciation: (ul"tru-fil'tur), [key]
— n.
  1. a filter for purifying sols, having a membrane with pores sufficiently small to prevent the passage of the suspended particles.
  2. a filter in a topological space, having the property that no other filter exists in the space having among its subsets all the subsets in the given filter.
—v.t.
  1. to purify by means of an ultrafilter.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.