period

期間、一回りの路(みち) ◇peri(＝周辺)＋hodos

(noun) An interval of time characterized by the occurrence of a certain condition, event, or phenomenon: a period of economic prosperity.
(noun) An interval of time characterized by the prevalence of a specified culture, ideology, or technology: artifacts of the pre-Columbian period.
(noun) An interval regarded as a distinct evolutionary or developmental phase: Picasso's early career is divided into his blue period and rose period.
(noun) Geology A unit of time, longer than an epoch and shorter than an era.
(noun) Any of various arbitrary units of time, especially:

The RE is, at least as I understand it (and in 40 years of statistics and econometrics I never ran in to it until started reading climate papers), is 1 minus the ratio of the predicted minus actual values (both in the verification period) squared over the squared deviations of the actual values (in the verification period) less the mean of the __calibration period__.