Waste, rubbish, trash, or garbage is unwanted or undesired material.

There are a number of different types of waste. It can exist as a solid, liquid, or gas or as waste heat. When released in the latter two states the wastes can be referred to as emissions. It is usually strongly linked with pollution. Waste may also be intangible in the case of wasted time or wasted opportunities. The term waste implies things which have been used inefficiently or inappropriately.

Some components of waste can be recycled once recovered from the waste stream, e.g. plastic bottles, metals, glass or paper. The biodegradable component of wastes (e.g. paper & food waste) can be composted or anaerobicly digested to produce soil improvers and renewable fuels. If it is not dealt with sustainably in this manner biodegradable waste can contribute to greenhouse gas emissions and by implication climate change .

There are two main definitions of waste. One view comes from the individual or organisation producing the material, the second is the view of Government, and is set out in different acts of waste legislation. The two have to combine to ensure the safe and legal disposal of the waste .

The UK's Environmental Protection Act 1990 indicated waste includes any substance which constitutes a scrap material, an effluent or other unwanted surplus arising from the application of any process or any substance or article which requires to be disposed of which has been broken, worn out, contaminated or otherwise spoiled; this is supplemented with anything which is discarded otherwise dealt with as if it were waste shall be presumed to be waste unless the contrary is proved. This definition was amended by the Waste Management Licensing Regulations 1994 defining waste as:

"any substance or object which the producer or the person in possession of it, discards or intends or is required to discard but with exception of anything excluded from the scope of the Waste Directive".