Having a mutual or reciprocal relationship. Fitting together and varying together. See text, Chapter 13. See also, "coefficient of correlation," "mean," "regression" and "standard deviation."
A parallel relationship between two markets. Markets which move in the same direction at the same time.
When investments are affected by the same economic pressures, they will rise or fall at the same time. If all your assets are correlated, you will lose much more money, thus increasing risk.