"Information retrieval is a wide, often loosely-defined term but in these pages I shall be concerned only with automatic information retrieval systems. Automatic as opposed to manual and information as opposed to data or fact. Unfortunately the word information can be very misleading. In the context of information retrieval (IR), information, in the technical meaning given in Shannon's theory of communication, is not readily measured (Shannon & Weaver). In fact in many cases, one can adequately describe the kind of retrieval by simply substituting "document" for "information". Nevertheless, "information retrieval" has become accepted as a description of the kind of work published by Cleverdon, Salton, Spark Jones, Lancaster and others. A perfectly straightforward definition along this line is given by Lancaster 2: "Information retrieval is the term conventionally, though somewhat inaccurately, applied to the type of activity discussed in this volume. An information retrieval system does not inform (i.e. change the knowledge of) the user on the subject of his inquiry. It merely informs on the existence (or non-existence) and whereabouts of documents relating to his request". This specifically excludes Question-Answering systems as typified by Winograd 3 and those described by Minsky 4. It also excludes data retrieval systems such as used by, say, the stock exchange for on-line quotations."