• In engineering - particularly control engineering - most systems are represented by a state vector. 

- The state vector contains all of the relevant quantities that describe the current state of the system being controlled.
- E.g. position, velocity, acceleration are standard quantities.
- However, the state vector could also include things like mass: for a burning rocket - which has an effect on the centre of mass
and hence the ability to control the flight of the system.

• In tracking problems - where the system being tracked may be somewhat un-cooperative - the state 

vector represents the state of knowledge about the system of interest.