Correlation analysis, correlation is a term that is a measure of the strength of a relationship between two variables.
Pearson’s Product-Moment Correlation
One of the most common measures of correlation is Pearson’s product-moment correlation, which is commonly referred to simply as the correlation, or just the letter r.
Correlation shows the strength of a relationship between two variables and is expressed numerically by the correlation coefficient.
Click through for examples from several packages. H/T R-Bloggers.