What is correlation?
correlation is Most generally, the degree to which one phenomenon or random variable is associated with, or can be predicted from, another. In statistics, correlation usually refers to the degree to which a linear predictive relationship exists between random variables, as measured by a correlation coefficient (q.v.). Correlation may be positive (but never larger than 1), i.e. both variables increase or decrease together; negative or inverse (but never smaller than -1), i.e. one variable increases when the other decreases; or zero, i.e. a change in one variable does not affect the other.
source: A GLOSSARY OF TERMS FOR COMMUNITY HEALTH CARE AND SERVICES FOR OLDER PERSONS. World Health Organization, 2004.