Components of a System
An entity is an object of interest in the system.
An attribute is a property of an entity. A given entity can possess many attributes.
An activity represents a time period of specified length.
The state of a system is defined to be that collection of variables (e.g. entities, attributes, activities) necessary to describe the system at any time, relative to the objectives of the study.
An event is defined as an instantaneous occurrence that may change the state of the system.
The progress of the system is studied by following the changes in the state of the system.