1. A computer-implemented method comprising: one or more computing devices associating a category to a set of nodes of a graph by: the one or more computing devices determining a first node that represents a first term that is in the category; the one or more computing devices locating a second node associated with the first node based at least in part on a first degree of cross-reference between the first node and the second node, the second node representing a second term, wherein the first degree of cross-reference is based at least in part on a frequency by which the first term appears in a set of documents with the second term; the one or more computing devices locating a third node associated with the second node based at least in part on a second degree of cross-reference between the second node and the third node, the third node representing a third term, wherein the second degree of cross-reference is based at least in part on a frequency by which the second term appears in a set of documents with the third term; based at least in part on both (a) the first degree of cross-reference between the first node and the second node, and (b) the second degree of cross-reference between the second node and the third node, determining whether or not the third term is in the category; in response to determining that the third term is in the category, storing information that indicates the third term is in the category.