What is a system?

Before entering this course, it would have been very difficult for me to answer this question, as there were lots of ambiguous definitions I would have used for "system". Now I can define a system as  a set of components that function and interact in a predictable manner. The components in a system are inputs, outputs, flows, sources, storages and sinks. In an ecosystem, for example, the sun is a source of energy. This energy flows to plants, which are a storage, and then flows again to herbivores. When herbivores die, the energy flows to the ground, which is a sink. Making some change in the ecosystem, such as introducing a new species, becomes an input, and the result of this change is known as the output and affects the whole ecosystem, such as less food available for the other species in the area. A system can be classified as open, closed or isolated. Open systems can transfer matter and energy in and out of their boundaries, while closed systems can only transfer energy. We can think of a forest as an open system and a greenhouse as a closed system. Isolated systems don't transfer energy or matter through boundaries, which is only a term because this can't happen. The universe is the only example of an isolated system because there's no other system to transfer matter or energy. It's good to finally have the question answered. :)