• 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 reciprocal relation between two or more things
• 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)

  Example: "What is the correlation between those two variables?"

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