From Computer History Wiki
Real-time might refer to things that are related to things which are happening in the 'real world', and happen in a short time-scale, in immediate response to those. Systems which are intended to provide this sort of response are called real-time systems.
Real-time is also used to describe programs (such as simulations) which have an internal idea of time, and in which time procedes internally at the same rate as in the 'real world' - i.e. in 'real time'.