Correlation

A measure of linear association between two independent continuous (a.k.a. variable or numerical) values. The nature of the relationship is where as one value changes, the other value moves at a predictable (yet not always equal) rate. It can be measured by a correlation coefficient (a.k.a. Pearson Correlation) and can be positive or negative. A positive correlation means the dependent factors move in the same increasing or decreasing direction as the independent value. A negative correlation means those dependent factors move at an inverse or opposite direction than the independent value. 
Related StatStuff Video: 
   -Section 6-Analyze Phase, Lesson #26 - Hypothesis Testing: Relationships (Overview)