What is "s.t.a.l.k.e.r."

S.T.A.L.K.E.R. is a series of first-person shooter survival horror video games developed by Ukrainian video game developer GSC Game World for Microsoft Windows. Based very loosely on the novel Roadside Picnic and the 1979 film adaptation Stalker, the games are set in the area surrounding the Chernobyl accident site, called "The Zone", in an alternative reality where a second explosion occurs at the Chernobyl Nuclear Power Plant some time after the first and causes strange changes in the area around it.