Stability and Fluid Flows
Fluid flows can be charaterized as stable or unstable.
If a flow is stable, and I perturb it slightly (I bump the experiment, for example) the flow field returns to its original state. If this is the case only for small perturbations (i.e. perturbations with velocities much smaller than those present in the original flow field) then the flow is called linearly stable. If it is also true for large perturbations then the flow is called unconditionally stable.
A flow is unstable if a perturbation results in a different flow field.