The extent to which two or more variables are related to each other, generally expressed numerically and referred to as a “correlation coefficient.”
The correlation coefficient is measured on a scale that varies from + 1 through 0 to – 1. Complete correlation between two variables is expressed by either + 1 or -1. When one variable increases as the other increases the correlation is positive; when one decreases as the other increases it is negative. Complete absence of correlation is represented by 0.
https://www.bmj.com/about-bmj/resources-readers/publications/statistics-square-one/11-correlation-and-regression