1. The quality or attribute of being insecure in functioning properly and to being irresolute; unreliability: Such issues as racial problems, economics, the concentration or density of population, and family instabilities can lead to violence and bloodshed.
2. A disposition toward unpredictable conduct or irregular changes in one's state of mind: The older Aunt Jane got, the more mental instability she showed and therefore had to have help in her home.

The abnormality or irregularity in a process involving the combustion of a source of energy or fuel: In aerospace engineering, combustion instability may take place in a rocket engine.

In classifying combustion instabilities there are three types which are related to engines: chamber instabilities, intrinsic instabilities, and system instabilities.

An incessant or unrelenting status of unwanted self-oscillation in an amplifier or other electric circuit: At school, Jim understood electrical instability to be a reaction relating to transistors with amorphous indium-gallium-zink oxide which were analyzed with the illumination of light.