What is "semivariance"

n. (context statistics English) A measure of the dispersion of those values that fall below the mean or target value of a data set

''For the measure of downside risk, see Variance#Semivariance

In spatial statistics, the empirical semivariance is described by

$$\hat\gamma(h)=\frac{1}{2}\cdot\frac{1}{n(h)}\sum_{i=1}^{n(h)}(z(x_i+h)-z(x_i))^2$$

where z is a datum at a particular location, h is the distance between ordered data, and n(h) is the number of paired data at a distance of h. The semivariance is half the variance of the increments z(x + h) − z(x), but the whole variance of z-values at given separation distance h (Bachmaier and Backes, 2008).

A plot of semivariances versus distances between ordered data in a graph is known as a semivariogram rather than a variogram. Many authors call $2\hat\gamma(h)$ a variogram, others use the terms variogram and semivariogram synonymously. However, Bachmaier and Backes (2008), who discussed this confusion, have shown that $\hat\gamma(h)$ should be called a variogram, terms like semivariogram or semivariance should be avoided.