One of the mysteries of the English language finally explained.
A diagram or method of graphical representation which is designed to detect or display any periodicity (usually with time) in a set of measurements; specifically one in which the results of harmonic analysis of the data, performed on the assumption in turn of different periods of variation, are plotted as a function of the period.
Late 19th century. From period + -o- + -gram. Compare slightly later periodograph.
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