Object Model

What is an Object Model?

the physical layout of objects in memory. E.g. the the COM ObjectModel, the JVM ObjectModel.
the nature of objects in a programming language (including everything objects inherit from Object). E.g. the Java ObjectModel or the Smalltalk ObjectModel.
a kind of diagram, or an incarnation of that kind of diagram. E.g. the UnifiedModelingLanguage ObjectModel diagram or the ObjectModel of an application domain.
a specific configuration of objects at runtime. E.g. if you program in JavaScript you have different ObjectModel-s in NetscapeNavigator and InternetExplorer.
The OMT methodology uses the term "object model" to refer the class diagrams (and instance diagrams) used to describe part or all of a system.