From Glossary of Meteorology


(Sometimes called depression.) In meteorology, an "area of low pressure," referring to a minimum of atmospheric pressure in two dimensions (closed isobars) on a constant-height chart or a minimum of height (closed contours) on a constant-pressure chart.

Since a low is, on a synoptic chart, always associated with cyclonic circulation, the term is used interchangeably with cyclone.
Compare high.