Q factor
also known as: Quality factor
How underdamped a resonant system is: high Q rings long and peaks sharply, low Q is sluggish and broad. For filters, Q sets the shape near the corner — 0.707 is maximally flat; higher peaks, lower droops.
In practice
In an LC circuit Q tells you the bandwidth (f0/Q) and the voltage magnification at resonance. In active filters, each cascaded stage has its own Q — and component spread grows with Q², which is why very sharp filters get awkward to build.