Correlation

Covariation of two or more variables; a relation between the values of two or more variables that is manifest in the variation of cooccurrances. In statistics, that form of an association in which an increase in the value of one variable tends to either increase the values of the other (positive correlation) or decrease the values of the other (negative correlation) rendering one variable (linearly or non-linearly) dependent on the other. (Krippendorff )