During the last years object-oriented database systems have become a major topic in database research. A large number of research institutes are working on individual aspects or complete system prototypes. However, there is still much confusion about which features make up such a system. This paper attempts to define the main features and characteristics. It distinguishes between a basic definition and an extended definition. The latter tries to cover some more extensive requirements for object-oriented database systems as they are introduced by non-standard applications. Moreover, the paper attempts to identify the most important variants by which a feature may be realized and shows some open fields for research work.