The straight line that best summarizes the distribution of data points on a bivariate scatter diagram by minimizing the sum of the vertical squared deviations of each data point from the regression line.
(Manheim, J.B., Richard, C.R., Willnat, L., & Brians, C.L. (2008). Empirical political analysis: Quantitative and qualitative research methods (7th ed.). New York: Pearson.)