From Glossary of Meteorology
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In meteorology, an elongated area of relatively low atmospheric pressure; the opposite of a ridge.

The axis of a trough is the trough line. This term is commonly used to distinguish the previous condition from the closed circulation of a low (or cyclone), but a large-scale trough may include one or more lows, an upper-air trough may be associated with a lower-level low, and a low may have one or more distinct troughs radiating from it.
See front, dynamic trough, easterly wave, equatorial wave.