The system of equations in (1) is an example of a * flow*,
which gives rise to a * continuous* evolution of ``field lines'' in
the **n**-dimensional space (sometimes called the ``phase space'')
[2].
We say that the flow is * conservative* if the volume in phase space
remains constant with time. If the volume decreases with time, there is a
damping effect in the dynamics, and we refer to the resulting flow as
* dissipative*.
(A frictional force in a mechanical system gives rise to dissipation, as
does a resistor in an electrical system.)

Also, it is sometimes convenient to write Eq. (1) in column vector form

where