Originally it came from considering the rate of change of technology. The rate of change of technology is generally considered to be increasing. Since the rate of change of technology seems to be increased by new technology, it is perhaps exponentially increasing. It's natural to ask: what is the maximum rate of change of technology? If that maximum is very high, then there may come a time when the change in technology that used to take years will take days, then hours, then just minutes, etc. The Singularity is the theoretical time that is asymptotic to the technology curve as the rate of change of technology tends to infinity. In less mathematical language, it is the day when hundreds of years of technology change occurs in hundreds of minutes - sometimes imagined as AI researchers working in time-accelerated virtual environments.