7.1.2—The terms displacement, amplitude, phase difference, period, frequency

Amplitude is maximum displacement; period T is time per cycle; frequency f is cycles per second with f=1/T; phase difference compares positions in their cycles.

Use the same reference point when comparing phase. Amplitude is not the wavelength, and frequency is not the wave speed.

A 5 Hz oscillator has period 0.20 s; two points half a cycle apart differ in phase by 180°.

Points can be in phase even when separated by one wavelength, and larger amplitude does not automatically mean higher frequency.