The flicker fusion threshold, also known as critical flicker frequency, flicker modulation threshold, or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is a concept studied in vision science, more specifically in the psychophysics of visual perception. The effect was traditionally called "persistence of vision", but that term has also been used to describe positive afterimages and motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.