piece of plastic microplastic particle microplastic envoPlastics A piece of plastic which has a diameter less than five millimetres. This classification is based on the thresholds detailed in GESAMP report # 99 on recommendation from the SDG 14 indicator team. While the definition is in accordance with the common use of this term (as per GESAMP report # 99), alternative size classifications exist. Alternative classifications are a piece of plastic which is between 1 and 5 millimetres in diameter, a piece of plastic which has a diameter less than 1 millimetre, or a piece of plastic which has a diameter greater than 330 micrometres. The use of the word diameter in this case was understood to mean the generalised form of diameter, i.e. the largest distance between two points belonging to an object.