Reaction videos are videos in which people react to events. In particular, videos showing the emotional reactions of people viewing television series episodes or film trailers are numerous and popular on video hosting services. The depicted persons may or may not be aware that they are being recorded, and the video being reacted to may or may not be reproduced within the reaction video, allowing the reaction video's viewers to see directly what is being reacted to.
One of the first viral reaction videos was that of a child reacting to the "Scary Maze Game" prank in 2006. Beginning in 2007, reaction videos began to proliferate on the Internet. Among their first topics were reactions to the fetish pornography trailer 2 Girls 1 Cup. By 2011, videos of people recording themselves reacting to film trailers had become a staple of services such as YouTube. The numerous reaction videos for particularly popular or shocking television events, such as the 2013 Game of Thrones episode "The Rains of Castamere", have themselves become the subject of commentary.
Sam Anderson, commenting on the phenomenon for the New York Times Magazine, described it as encapsulating the "fundamental experience of the Internet" in that it involved watching screens on which people watched screens, in a potentially infinite regression. The first reaction videos for the gross-out "2 Girls 1 Cup" allowed people, according to Anderson, to "experience its dangerous thrill without having to encounter it directly — like Perseus looking at Medusa in the reflection of his shield". But much like the later videos featuring reactions to items of popular culture, Anderson wrote, such videos provide the appeal of experiencing, "at a time of increasing cultural difference, the comforting universality of human nature" in showing people of all backgrounds reacting similarly to a shared cultural experience. In CraveOnline, Witney Seibold derided reaction videos as "graceless" and "narcissistic", because they merely reflected immediate emotional reactions, and doubted that the reactions of a person aware of being filmed could in fact reflect the honest emotional response promised by the format.