An Etymological Dictionary of Astronomy and Astrophysics

Physics: The duration of one complete cycle of an oscillation;
the reciprocal of the frequency.
Astro.: The amount of time an astronomical body takes to complete an orbit.

Etymology (EN): From M.E. periode, from M.Fr., from M.L. periodus “recurring portion, cycle,” from L. periodus “a complete sentence,” from Gk. periodos “rounded sentence, cycle, circuit, period of time,” literally “going around,” from → peri- “around” + hodos “way, journey;” cognate with L. cedere “to go, yield.”

Etymology (PE): Dowré, from dowr, from Ar. daur “age, time; revolution.”