In real-life, there are many situations where two variables depend not just on each other but also on another variable, such as time. One classic example is population growth of two species where one is a predator of the other. In this example, the population of prey impacts the population of predators through food availability, the population of predators impacts the population of prey through hunting, and both populations also change proportionate to time. The populations are interdependent.

We call these coupled systems.