Root locus, introduced by W.R. Evans in 1948, is a technique used to adjust the location of closed-loop poles for desired system performance by varying parameters, primarily for stability analysis in control systems. The document outlines the procedure for constructing root locus diagrams, including key concepts like poles, zeros, centroids, asymptotes, breakaway points, and intersections along the imaginary axis. It emphasizes the importance of sketching the root locus in the context of unity feedback control systems.