In information retrieval, a measure of search effectiveness, expressed as the ratio of relevant records or documents retrieved from a database to the total number retrieved in response to the query; for example, in a database containing 100 records relevent to the topic "book history," a search retrieving 50 records, 25 of which are relevant to the topic, would have 50 percent precision (25/50). Synonymous with relevance ratio. Compare with recall. See also: fallout.

From ODLIS, Online Dictionary of Library and Information Science by Joan M. Reitz
http://lu.com/odlis/odlis_c.cfm