The document discusses the silhouette coefficient, which is used to measure how closely grouped objects are within their assigned cluster. It defines the silhouette coefficient s(i) for each object i based on comparing a(i), the average similarity of i to other objects in its assigned cluster A, and b(i), the average similarity of i to objects in the next best cluster B. Values of s(i) near 1 indicate i is well matched to cluster A, while values near -1 suggest i belongs more in cluster B. The silhouette coefficient provides a way to validate how appropriately objects have been clustered.