The term is 20 years old and still doesn't have one universal size cutoff — here's the definition this Standard actually uses.
"Microplastics" entered the scientific vocabulary in 2004, when a paper in Science described widespread microscopic plastic fragments and fibers accumulating in ocean sediment and the water column [1]. The term stuck, but a precise size definition took another two decades to standardize.
The most current cross-matrix framework, published in 2023, defines microplastics as solid, water-insoluble plastic particles with at least one dimension between 1 micrometer (µm) and 1,000 µm [2]. Below 1 µm, particles are generally classed as nanoplastics — a distinction that matters more than it sounds, because it determines which analytical method can even see them (see the companion article on µFTIR, Raman, and Py-GC-MS).
Different regulatory and research bodies have historically used slightly different upper bounds (5 mm is common in marine debris literature), and the 1–1,000 µm range itself isn't a claim about biological relevance — it's a practical range chosen because it's what current instrumentation can reliably size and count.
This is also why the MPFREE Standard doesn't publish a single "parts per billion" number for microplastic content (see the certification-and-standards articles): a single mass-based unit can't describe both a 900 µm fragment and a 2 µm fiber in the same breath. The Standard instead specifies a threshold per analytical method, each with its own reporting unit and size floor.