WordNet sense 1 (a reciprocal relation between two or more things):

correlativity

WordNet sense 2 (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)):

correlation coefficient, coefficient of correlation

WordNet sense 3 (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):

correlational statistics