A system is steady-state if its properties - pressure, velocity, etc - do not change with time. Any disturbance to a steady-state system will cause these properties to change - this change is a fluid transient.
Note: Fluid transients and their effects are commonly referred to with the term waterhammer. While the term is most appropriate for water systems, waterhammer analysis applies to any liquid.
All fluid transients cause disturbances which propagate throughout the fluid as coupled pressure and velocity waves. This behavior is perhaps best explained with a conceptual example. Like any acoustical wave, these waves reflect, combine, and dissipate. This behavior can be extremely complex, even in relatively simple systems.
Slow acting system changes - such as a reservoir level that changes over the course of a day - can be accurately approximated with a series of steady-state simulations. There are still rapidly-propagating transient pressures in the system, but their magnitude is generally small enough to be considered negligible.
Rapid changes can cause large transient pressures that propagate through the piping network. In extreme cases this causes catastrophic failure of piping and associated equipment. AFT Impulse is focused on analyzing rapid changes to the state of a fluid piping system and the resulting transient behaviors via the Method of Characteristics.