Finding definitions in huge text collections is a challenging problem, not only because of the many ways in which defini-tions can be conveyed in natural language texts but also be-cause the definiendum (i.e., the thing to be defined) has not, on its own, enough discriminative power to allow selection of definition-bearing passages from the collection. We have de-veloped a method that uses already available external sources to gather knowledge about the “definiendum ” before trying to define it using the given text collection. This knowledge consists of lists of relevant secondary terms that frequently co-occur with the definiendum in definition-bearing passages or “definiens”. External sources used to gather secondary terms are an on-line enyclopedia, a lexical database and the Web. These secondary terms together with the definiendum are used to select passages from the text collection perform-ing information retrieval. Further linguistic analysis is car-ried out on each passage to extract definition strings from the passages using a number of criteria including the presence of main and secondary terms or definition patterns.