it is the assumption that the random errors have zero mean and the covariance is the same between any two points that are at the same distance and direction no matter which two points you choose.
SECOND-ORDER STATIONARITY
Meaning of SECOND-ORDER STATIONARITY in English
Sean Woo's Finance, GIS and real estate English glossary. Английский глоссарий по ГИС(Географическая информационная система) и недвижимости Sean Woo's Finance. 2012