persistence

1. The previous value in a time series.

Thus, if x(t) denotes the present value, the value of persistence would be x(t - 1), whence the latter value is regarded as "persisting." It is used as an objective standard in the verification of weather forecasts. Sometimes it is extended to mean x(t - h) where h is arbitrary.

1. (Also called constancy, steadiness.) With respect to the long-term nature of the wind at a given location, the ratio of the magnitude of the main wind vector to the average speed of the wind without regard to direction.
   
2. (Also called inertial forecast.) In general, the tendency for the occurrence of a specific event to be more probable, at a given time, if that same event has occurred in the immediately preceding time period.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.