Abstract: Compound terms are essential in technical terminologies because of the need to distinguish precisely between subtle or detailed differences in concepts or things. Compound terms of different sorts require different approaches in defining and in handling. Unitary compound terms, those consisting of a base word and one or more affixes, are common in nontechnical usage, but are generated in technical terminological work by drawing from a greater number of less common affixes. Still, multiple meanings for the affixes are common. As in the case of ordinary usage, some affixes are independent enough so that the affix may be defined, but the compound term does not require definition. Indexing may require cross referencing in some cases.Compound terms consisting of more than one word are common in most technical fields. These terms may be classified as one of three types; sentential, hierarchic, or categoric. Sentential compound terms are descriptive and incidental, for example two-inch stirred chemical reactor. Such terms rarely require definition and usually would be further described in the context of the term's usage. Hierarchic compound terms from a mini-classification and may be actually precursors of elements of a formal classification. These terms require definition when the implied classification is essential but not formalized. In this case a vocabulary or terminology provides a convenient display of part of a nonincidental aspects of the concept or entity may be viewed as a faceted (nonhierarchic) classification. The style of definition and the indexing of technical terms may be most efficiently approached with a knowledge of the varieties of compound terms.