1. A set of interrelated elements separated from the environment (adapted from DIN IEC 60050-351)
2. Note: A system is characterised by:
2.1. the definition of a borders to the environment (system borders), with which it can exchange material, energy and information via interfaces (system input and output).
2.2. its elements which, when the resolution is increased, represent systems themselves (subsystems) or which are regarded as not being decomposable any further (system elements).
2.3. the sequence structure in the elements which is characterised by specific rules and constant or variable attributes.
2.4. the relations which connect the system elements among each other (composition structure) so that a process can be run.
2.5. the states of the elements which are each described by stating the values of all constant and variable attributes (state variable) of which generally only a small part is relevant to the investigation.
2.6. the state transitions of the elements as continuous or discrete changes of at least one state variable due to the process running in the system.