In statistics, the strength of a relationship between two variables. The relationship can be positive (when the value of one variable increases, the other also increases) or negative (when the value of one variable increases, the other decreases). Correlation does not imply causality; a third factor could instead be the cause of an apparent causal relationship between two variables.
Example: The taller you are, the more you are likely to weigh. Therefore, height and weight are positively correlated.
Grinnell, R. (2008). Correlation. Psych Central. Retrieved on April 29, 2016, from http://psychcentral.com/encyclopedia/correlation/