correlation

(noun) a reciprocal relation between two or more things. Synonyms: correlativity.

(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). Synonyms: correlation coefficient, coefficient of correlation. Example: "What is the correlation between those two variables?."

(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. Synonyms: correlational statistics.