Periodical, something that terminates or comprehends a period; as a periodic month; or the space of time in which the moon completes her period.

Perioeci (περιοίκοι), in Geography, such inhabitants of the earth as have the same latitudes, but opposite longitudes, or live under the same parallel and the same meridian, but in different semicircles of that meridian, or in opposite points of the parallel. These have the same common seasons throughout the year, and the same phenomena of the heavenly bodies; but when it is noon-day with the one, it is midnight with the other, there being twenty-four hours in an east or west direction. These are found on the globe by the hour-index, or by turning the globe half round, that is, 180 degrees either way.

Peripatetics, philosophers, followers of Aristotle, and supporters of the peripatetic philosophy, called also Aristotelians. Cicero says that Plato left two excellent disciples, Xenocrates and Aristotle, who founded two sects, which only differed in name; the former took the appellation of Academics, being those who continued to hold their conferences in the Academy, as Plato had done before; the latter, who followed Aristotle, were called Peripatetics, from περιπατέω, I walk, because they disputed walking in the Lyceum. (See Aristotle and Aristotelian Philosophy.)