International Encyclopedia of Systems and Cybernetics

Any unnecessary, useless, or nocive output of a system.

There are many different types of wastes. However most kinds can be classified as a useless output of either matter, or energy or information.

The sore point in the definition is the "useless" adjective. In many cases, it really means that we do not see any use for that specific output. In natural systems however, every output is finally recycled and again transformed into a resource, with the exception of final residual heat.

As to now, organic waste is generally recycled through biodegradation (but this process may take a long time and excesses of organic waste may asphyxiate some ecosystems). Inorganic waste, generally of artificial industrial and domestic origin, is mostly non-degradable, at least for an indefinitely long period, and tends thus to have these suffocating side-effects.

Many artificial wastes, produced by human systems, could possibly be recycled (and should be, whenever "practically" – another doubful adjective – feasible).

A good general understanding of the various kinds of waste flows and their actual as well as possible alternative circuits, is the first step toward this end.

H.T. ODUM writes: "The cheapest way would be to use man's waste flows to stimulate the circuits of his interest" (1971, p.290).

He adds that, when various possibilities of recycling do exist, value decisions should be based on the most efficient use of energy.(in thermodynamic terms).

Unfortunately, this still remain mostly wishful thinking because, in the words of E.J. KROWITZ: "How to measure entropy content; how to model the economic transformation in entropy terms, and measuring the environmental- resource limits all pose major obstacles to implement the concept" (1991, p.45).