This word has tons of definitions. So which Period are you talking about exactly?
PHYSICS
the interval of time between successive occurrences of the same state in an oscillatory or cyclic phenomenon, such as a mechanical vibration, an alternating current, a variable star, or an electromagnetic wave.
ASTRONOMY the time taken by a celestial object to rotate around its axis, or to make one circuit of its orbit.
MATHEMATICS the interval between successive equal values of a periodic function.
A flow of blood and other material from the lining of the uterus, occurring in nonpregnant women at intervals of about 28 days between puberty and menopause and typically lasting for a few days.
