APA Dictionary of Psychology

n. a precise statement about how two pieces of information, usually mathematical, are related. The most common type of formula is a mathematical equation in which one variable is a weighted function of another variable plus a constant. For example, the formula for a straight line is y = mx + b, where y is an outcome plotted on a vertical or y-axis, m is the slope of the line, x is the value of a variable along the horizontal or x-axis, and b is where the straight line crosses the vertical axis.