nadir
origin.
apogee
apex
Origin, source
zero
nadir
The base of a wave is the lowest point of the wave where it meets the horizontal axis. It represents the starting point of the wave's oscillation.
The starting point of a thermometer is typically the lowest end of the temperature scale, which is usually labeled as 0 degrees Celsius or 32 degrees Fahrenheit, depending on the type of thermometer.
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Trough. The crest is the highest point of the wave while the trough is the lowest point.