Real-time simulation refers to a computer model of a physical system that can execute at the same rate as actual "wall clock" time.
Real-time simulation occurs commonly in computer gaming, but also is important in the industrial market for operator training and off-line controller tuning.
Other techniques having variable step are used for high frequency transients but are unsuitable for real time simulation.
In simple words, real-time simulation must produce the internal variables and output within the same length of time as its physical counterpart would.
Configuring models to run in real-time enables one to use hardware-in-the-loop simulation to test controllers.