In this video, Dr. Walter Lewin presents the concepts of period and frequency related to circular motion. Period (T) is the time it takes for a repeating event or motion to complete one full cycle, or oscillation. Examples include the time for Earth to complete one orbit around the Sun (one year). Frequency (f) is a measure of how often a repeating event or wave cycle occurs within a specific unit of time, usually one second. It is measured in hertz (Hz), where 1 Hz equals one cycle per second.