A quantity that repeats — a swing, a tide, a heartbeat — is described by three numbers, and their units keep them apart. The period $T$ is the time one full cycle takes, in seconds. The frequency $f$ counts how many cycles fit into one second, in hertz — read aloud: one hertz is one cycle per second. And the amplitude is how far the motion leans from its rest position — a length, measured from the middle, never from crest to trough.

Notice what the units confess before any formula arrives: seconds mean someone TIMED one cycle; hertz mean someone COUNTED cycles for a second; metres mean someone measured a reach. One motion, three measurements — and every periodic function this chapter builds runs on them.