underfilter noun 2verb 1forms 3 noun2 •A filter that is positioned beneath something.Since there is no underfilter, the top flow line emerges on the downstream slope. — 1988 •A morphological filter (idempotent operator) whose image is a subset of the domain.It is clear that overfilters and underfilters are dual in the sense of the duality principle. — 1999 verb1 •To filter insufficiently.It is not possible to overfilter your tank but marine aquaria are frequently underfiltered. — 1966 formsunderfiltered · underfiltering · underfilters