Water Hammer and Hydrodynamics’ Instability - Advances in Control and Automation of Water Systems - page 64

figure 4.1 Newton second law-forces analysis for fluid element.

figure 4.2 Continuity equation or conservation of momentum.

Those Equations can be expressed graphically in space-time as characteristic lines.
Theses lines are called characteristics lines. (Figure 4.3-4.5) Theses lines represent
signals propagating to the right (C+) and to the left (C-) simultaneously and from each
location in the system.