A dendrogram is a tree-like diagram that visually represents the arrangement of clusters or groups in data analysis. It is commonly used in fields like biology for displaying evolutionary relationships among species, and in data science for illustrating hierarchical clustering results. Each branch of the dendrogram indicates a relationship between data points, with the length of the branches often representing the distance or dissimilarity between them. In a dendrogram, the vertical axis typically shows the distance or dissimilarity, while the horizontal axis lists the individual items or clusters. As you move up the diagram, clusters merge, demonstrating how closely related the items are. This makes dendrograms useful for identifying patterns and relationships in complex datasets.