Heteroskedasticity

http://en.wikipedia.org/wiki/Heteroscedasticity

The term means "differing variance" and comes from the Greek "hetero" ('different') and "skedasis" ('dispersion').

In statistics, a collection of random variables is heteroskedastic (often spelled heteroscedastic,^[1] and commonly pronounced with a hard k sound regardless of spelling) if there are sub-populations that have different variabilities from others. Here "variability" could be quantified by the variance or any other measure of statistical dispersion. Thus heteroscedasticity is the absence of homoscedasticity.