In statistics, a correlation is a value that defines a relationship between two variables so that it shows how one of the variables is tied (related) to the other.
The linear correlation coefficient, represented as r, defines the strength of the correlation between two elements of a sample. The elements are usually represented with x and y. This almost means that a value is considered as a pair of x and y.
The linear correlation coefficient is also called the Pearson product moment correlation coefficient (from Karl Pearson (1857-1936)).