Stability

The term stability is often used by engineers when they are talking about oscillators. In an oscillator, stability has 2 meanings: constancy of frequency and reliability of the performance. Evidently, both of these considerations are significant in the design of a good oscillator circuit.

Constancy of frequency
The foregoing oscillator types allow the frequency adjustment by using variable capacitors or variable inductors. The component values can be affected by temperature, and sometimes by humidity. When designing a variable-frequency oscillator (VFO), it is crucial that the components  maintain constant values, as much as possible,  under all the anticipated conditions.

Some types of the capacitors maintain their values better than other types, when the temperature goes up or down. Among best are polystyrene capacitors. Silver-mica capacitors also work well when the polystyrene units cannot be found.Inductors are most temperature stable when they are having air cores. They should be wound, when possible, from stiff wire with the strips of plastic to keep windings in place. Some air core coils are wound on the hollow cylindrical cores, which is made of ceramic or phenolic material. Ferromagnetic solenoidal cores are not very good for VFO coils, because the materials change their permeability as the temperature varies. This changes inductance, in turn affecting the oscillator frequency.

Engineers spend time and effort in finding components which will minimize drift (surplus changes in frequency over time) in VFOs.