APA Dictionary of Psychology

n. a formal, usually brief, statement showing that two expressions are the same or equal. In mathematics or statistics, an equation is often a statement with y on the left of an equal sign and an expression for delineating or modeling y on the right side. For example, a commonly used equation for a straight line is y = mx + b, where m stands for the slope of the line, x stands for different values along the x-axis, and b stands for the point where the line crosses the y-axis.