Correlation Coefficient

The correlation coefficient (usually written r) is a single number between -1 and +1 that measures how tightly two numeric variables move together in a straight-line pattern. An r near +1 means that when one goes up the other almost always goes up too; an r near -1 means one goes up while the other goes down; and an r near 0 means there is no straight-line relationship.

It is popular because it is unitless, so you can compare the relationship between height-and-weight against the relationship between study-hours-and-grades on the same scale. The big trap: correlation only sees straight lines, so a strong curved relationship can show r near 0, and a high r still does not prove that one variable causes the other (correlation is not causation).