Biodegradable plastic will degrade as a result of bacterial activity. In order to also be classified compostable, the degradation must meet certain specified criteria such as rate biodegradation, maximum residue of material left at a specific point in time and a requirement for the material to have no harmful impact on the final compost or the composting process.

Degradable plastic includes all classes of degradable plastic including the biodegradable and compostable. However, plastic that is not biodegradable or compostable usually use the label Degradable plastic. Most of the products using the label Degradable plastic, degrade as result of physical and chemical impact (fracture into smaller pieces of plastic). Biological activity is not a significant part of the degradation of these products, or the process is too slow to earn the classification Biodegradable or Compostable (the biological degradation will typically take many years).

ASTM and ISO have the following definitions on various degradable plastics:

degradable plastic , a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification.

biodegradable plastic , a degradable plastic in which the degradation results from the action of naturally-occurring micro-organisms such as bacteria, fungi and algae.

compostable plastic, a plastic that undergoes degradation by biological processes during composting to carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with other known, compostable materials and leaves no visually distinguishable or toxic residue.

photo degradable plastic , a degradable plastic in which the degradation results from the action of natural daylight.

oxidative degradable plastic , a degradable plastic in which the degradation results from oxidation.

hydrolytic degradable plastic, a degradable plastic in which the degradation results from hydrolysis.