Final answer:
A trough is the lowermost position of a wave, where the displacement of the medium is at a minimum.
Step-by-step explanation:
A trough is the lowermost position of a wave. It is the point on the wave where the displacement of the medium is at a minimum. In other words, it is the lowest point the medium sinks to.
For example, if you imagine a water wave, the crest would be the highest point where the water rises, and the trough would be the lowest point where the water sinks.