Answer:

In probability theory and statistics, correlation, (often measured as a correlation coefficient), indicates the strength and direction of a linear relationship between two random variables.

For e.g. Tall people usually have larger arm spans than shorter people. There is said to be a correlation between height and arm span, although there is no exact formula connecting them.

When two variables are such that as one changes the other changes in a related way, they are said to be correlated.