A dendrogram is a diagram of a tree, as per mathematics' graph theory. It is the familiar type of diagram seen showing family trees such as in genealogy or in evolutionary biology.
Dendrograms are used as one way to view astronomical data that has a hierarchical aspect and they are of current interest as a means to view hierarchical aspects of intensity mapping data, i.e., of grouping together the more-intense regions that are close to each other. In this case, for example, two small regions of high intensity spatially residing within a larger region of high-but-somewhat-less intensity would be "leaves" branching from a node indicating the larger region. And so on, hierarchically. Such trees (and dendrograms) can be constructed algorithmically.
In the study of molecular clouds and star formation, hierarchical trees are created from intensity data of a molecular hydrogen tracer such as carbon monoxide, which can be displayed as dendrograms. This can be done for such data mapped in position-position-velocity space, providing clues toward theorizing the actual intensity in ppp-space, i.e., locating molecular clouds, and their denser parts, such as dense cores. The tree-structured data also offers the possibility of (further) automating analysis, e.g., analyzing large amounts of astronomical survey data into star-formation-relevant information.