What term describes an unwanted oscillation in an amplifier or oscillator circuit?

Parasitic oscillation is the correct term for an unwanted, self-sustaining oscillation in an RF amplifier or oscillator. These occur at frequencies entirely unrelated to the desired operating frequency and can waste power while causing harmful interference. They are typically caused by poor circuit layout, where stray inductance and capacitance create an accidental resonant circuit within the amplifier. Distractors like 'harmonic emission' are incorrect because harmonics are exact integer multiples of the fundamental frequency, whereas parasitic oscillations are random, independent frequencies. 'Frequency instability' simply refers to an oscillator drifting off its intended frequency, not a separate unwanted signal.