Closed Low: A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see cutoff low).
Cutoff Low: A closed low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression). "Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.
Source: National Weather Service
closed low—A low that may be completely encircled by an isobar or contour line.
(This means an isobar or contour line of any value, not necessarily restricted to those arbitrarily chosen for the analysis of the chart.) Strictly, all lows are closed. However, in weather-map analysis terminology, this designation is used commonly in two respects: 1) on surface charts, to distinguish a low from a trough, especially as a low develops within the trough; and 2) on upper-level charts, to accentuate the fact that the circulation is closed, especially at levels and over latitudes where such an occurrence is unusual. The definition of closed high is analogous.
Source: AMS
