Here's a breakdown:

* Crest: The highest point of a wave.

* Trough: The lowest point of a wave.

Wavelength is typically measured in units of length, such as:

* Meters (m)

* Centimeters (cm)

* Nanometers (nm)

* Micrometers (µm)

The choice of unit depends on the scale of the wave being measured. For example, visible light has wavelengths in the nanometer range, while radio waves can have wavelengths of hundreds of meters.