correlation is the degree to which two (2) variables are associated.
In a Positive correlation, the two (2) variables increase or decrease together while in a Negative correlation, one variable decreases as the other increases.
Other /More definition:
The existence of a consistent, systematic relationship between two (2) events, measures, or variables; the degree to which two (2) variables are associated. In a positive Correlation, the two (2) variables increase or decrease together. In a negative correlation, one variable decreases as the other increases.
Moreover, Correlation is a statistical measure of the relationship between two (2) variables; statistical procedure used to measure the relationship between two variables.