ISO 1998 gives a list of equivalent English and French terms in use in the petroleum industry, accompanied by the corresponding definitions in the two languages. It was compiled to serve an evident need for a ready form of reference document. It therefore does not include all the possible terms, those terms of which significance is unambiguous being excluded. ISO 1998 is intended to cover the purposes of the part of petroleum industry dealing with crude oils and petroleum products, that means all related operations arising from the production field to the final user. It is not intended to cover either petroleum equipment, or any operation in the field. However, some pieces of equipment or some operations of exploration and production are defined. The corresponding terms were introduced only when they appear in a definition of a product or process and when their definition was found necessary for understanding or for avoiding any ambiguity. Where a terminology of petroleum equipment is needed, it corresponds to the scope of ISO/TC 67, Materials, equipment and offshore structures for petroleum and natural gas industries. The definitions are proposed for use in national and international standard documents. However, they are not intended as legal definitions. This part of ISO 1998 explains the classification system of categories and the numbering system of terms. It gives a general index.