Question

the features of a wave are listed below.
◆ crest: the highest point of a wave
◆ trough: the lowest point of a wave
◆ wavelength: the distance from one point on a wave to the same point on the next wave, such as from crest to crest or from trough to trough. a wave with a long wavelength carries less energy than a wave with a short wavelength.
◆ amplitude: the height of a wave crest from its rest position. amplitude is also the depth of a wave trough, as measured from its rest position. a wave with a large amplitude carries more energy than a wave with a small amplitude.
how many times does a wave repeat in a period of time?
another important way to describe a wave is by its frequency. you may have already heard the term frequency used to describe radio stations or music. the frequency of a wave is the number of times it repeats in a given period of time. a repetition of a wave is also called an oscillation or vibration. one vibration occurs over one wavelength.
frequency is measured in hertz. hertz (hz) means cycles per second.
when the wavelength is short, the frequency is high. when the wavelength is long, the frequency is low.
what are the different types of waves?
you have read that sound travels by sound waves. sound can travel through air, water, and even solid walls. the matter a wave travels through is called a medium. the medium can be a solid, liquid, or gas. for example, the medium of a water wave is water.
not all waves need a medium. for example, visible light waves and radio waves can travel through space where there is no matter.

Explanation:

The text states that "The frequency of a wave is the number of times it repeats in a given period of time" and "Frequency is measured in hertz. Hertz (Hz) means cycles per second." So, the frequency of a wave is measured by counting the number of times the wave repeats (or the number of oscillations/vibrations) in one second (since 1 hertz = 1 cycle per second).

Answer:

The frequency of a wave is measured by counting the number of times the wave repeats (or the number of oscillations/vibrations) in one second.