A function describing the degree of spatial dependence of a spatial random field or stochastic process

The variogram is defined as the variance of the difference between field values at two locations (x and y) across realizations of the field (Cressie 1993). A variogram will give a measure of how much two samples taken from an area will vary in some degree of a variable measured depending on the distance between those samples: samples taken far apart will vary more than samples taken close to each other.