Definition of "Correlation" :
noun: a statistical relation between two or more variables such that systematic changes in the value of one variable are accompanied by systematic changes in the other
noun: a statistic representing how closely two variables co-vary; it can vary from -1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation)
"What is the correlation between those two variables?."
noun: a reciprocal relation between two or more things