System is an object consisting of an aggregate of elements, which interact with each other and form a certain wholeness, or, unity. For example, an animal organism presents a system consisting of interacting elements - organs. The assamblage of chemical components (elements) of an organism, which are associated to each other by their interconversion (interaction), form a metabolic system.

Elements are parts of a system, which possess certain properties. As the result of dividing into elements, a hierarchy of systems of decreasing degree of complexity appears. Speaking of biological objects, the row is the following: biosphere, biocenosis, population, organism, organ, tissue, cell, organelle, molecule. An element of each level of complexity is also a system consisting of less complex elements.

There are two types of systems: open and closed systems. In a closed system its mass can neither increase or decrease during a change in a system, which is called a process, though energy can either decrease or come in.

If the composition and properties of system do not change during rather a long period of time, the system is considered to be in balance.

A system is homogeneous if its properties are the same in all its parts, and if it is continuous point-to-point. A system is heterogenous if it includes two or several separate regions, which are called phases and separated from each other by surfaces called surfaces of discontinuity.