trough
1.
2. A gutter under the eaves of a roof.
3. A long, narrow depression, as between waves or ridges.
4. A low point in a business cycle or on a statistical graph.
5. Meteorology. An elongated region of relatively low atmospheric pressure, often associated with a front.
6. Physics. A minimum point in a wave or an alternating signal.
[Middle English, from Old English trog.]