This chart displays a very basic, generalized view of cycles. Note that the tops of the cycles are called crests or peaks, and the bottoms of cycles are called troughs.
The length of a given cycle is called a period (or duration) and is measured from one trough to the next. This is not only due to established convention, but statistically, it has been shown that the time period between troughs has been found to be more consistent and reliable than the time period between crests. Period is, of course, measured in whatever unit of time is most appropriate, given the scale.