1 [computing] In object-oriented programming, a single object created based on the template or definition of the class to which it belongs.
2 [ESRI software] In ArcSDE, a single installation of ArcSDE associated with a single database.
3 [ESRI software] In ArcIMS, the fundamental processing unit of the Spatial Server. An instance takes a request and generates a response that can be sent back to a client.