In the natural or hard sciences, a closed system is one that does not interact with others. There are also open and isolated systems.
An example of a closed system is a balloon that is heated so that the gas inside makes it expand. This is a closed system because there is no exchange of matter; just the effect of the heat (energy) on the balloon. Another example would be a piston that responds to changes in heat.
By way of contrast, an open system involves the exchange of both matter and energy. Performing a titration, or using a known solution to determine the matter of an unknown solution, is an example of an open system with the beaker serving as the system.