The tendency of the fluid in the pump and piping to stay in motion unless acted upon by an outside force (friction in the pipes or a valve in the line will create this outside force), or to stay at rest unless acted upon by an outside force (starting the pump).

The practical application of this principle is that if you lose power to a running pump, inertia keeps the fluid flowing while the pump comes to a stop.

The fluid’s inertia can cause the fluid to separate in the piping and produce a water hammer when the fluid joins together again. This joining together of the fluid takes place at the speed of sound in the fluid you are pumping