  These notes are intended as an elementary introduction to the concept of
absolute instability. The transition from convective instability to absolute
instability is an important issue when the stability of stationary flow
solutions is investigated. The arguments here described were first developed in
the framework of plasma physics and later applied to the hydrodynamics of
mixing layers and shear flows. Far from being a comprehensive analysis of this
complicated subject, the aim of these notes is just to sketch a rudimentary and
quite elementary ground for physicists or engineers which have a familiarity
with the basic features of linear stability in fluid dynamics, but are new to
the concept of absolute instability.
