An optical effect caused by light rays constructively and destructively interfering. This is the same effect as why stars twinkle in the night sky, except that instead of optical light traveling through the Earth’s atmosphere, we observe radio waves traveling through the interstellar medium. This causes the pulsar to appear brighter at some times (and frequencies) and dimmer at other times, thus the pulse signal-to-noise ratio will vary. Scintillation occurs on different timescales, short timescales of order the length of our observations typically, and then long timescales of order days to years, from two separate regimes.