ASMR is a sound-triggered body experience: whispering, tapping, and careful attention sounds produce a static-like tingle over the scalp and a strong sense of calm in people susceptible to it. A very large online video economy exists because of it.
The phenomenon resembles a population-scale cousin of auditory-tactile synesthesia, with sound producing located physical sensation, though ASMR is mood-linked, trigger-broad, and not universal even among fans of the videos.
Research is young but growing: susceptibility varies stably between people, and physiological studies show measurable heart-rate reduction during episodes. The name is community coinage from 2010, adopted by the literature since.