A cluster is a subset of the complete network, a group of nodes and links which share a common concept and may be seen as an aggregate. For example, take this study. The nodes which explicate hypertext terms may be seen as a "definition cluster." Those dealing with Cortázar could be clustered under his name.
Some systems have mechanisms for making clusters explicit. In InterMedia, a folder holds a set of nodes; in StorySpace, nodes (here called workspaces) may be placed within each other, effectively creating aggregates.
Clusters provide a means of specifying inclusion or "belongs-to" relationships without using links. This is an important requirement of any sophisticated hypertext system. Links imply strong relationships while clusters allow for looser conglomerates. Typed links may be used to indicate clusters, but this method usually becomes unwieldy.
Writers such as Halasz have noted the need for a useful implementation. "The basic hypermedia model lacks a composition mechanism, ie., a way of representing and dealing with groups of nodes and links as unique entities separate from their components" (843). We prefer the less ambiguous term "cluster" to "composite" because to many "compose" equals "to author."