Frequency

Before jumping into frequency, we should touch on what it means to be "periodic," since that is closely related to frequency. Something (for example, an electrical signal or a motion) is considered periodic if it repeats itself after a period of time. Frequency is how often a periodic thing repeats itself in a given amount of time. These repetitions are often called "cycles" or "revolutions".

For example, if you have an electrical signal that repeats itself once per second, then that signal can be said to have a frequency of one cycle per second. (also call Hertz, Hz) However, there's nothing that says you have to use the second as your unit of time; you could also say that that signal has a frequency of sixty cycles per minute. Both are true, because there are sixty seconds in one minute.

As another example, think about the Earth revolving around the Sun. What do you think the frequency of the Earth around the Sun is? Probably the most straightforward way of representing the frequency of the Earth around the Sun is one revolution per year.