Definition (noun) a reciprocal relation between two or more things
Definition (noun) a statistic representing how closely two variables covary; it can vary from 1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation)
Example Sentence

what is the correlation between those two variables?
Definition (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