correlation coefficient

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A measure of the interdependence of two random variables that ranges in value from −1 to +1, indicating perfect negative correlation at −1, absence of correlation at zero, and perfect positive correlation at +1.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun statistics Any of the several measures indicating the strength and direction of a linear relationship between two random variables.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a statistic representing how closely two variables co-vary; it can vary from -1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation)