a period of about 19 years (almost 235 lunar revolutions), in which the phases of the moon repeat on the same dates as in the previous period: used for finding the date of Easter
See Metonic cycle in American Heritage Dictionary 4
(mĭ-tŏnˈĭk)
noun
A period of 235 lunar months, or about 19 years in the Julian calendar, at the end of which the phases of the moon recur in the same order and on the same days as in the preceding cycle.