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A nanoparticle is a particle whose external dimensions lie on the nanoscale. Its behaviour can depend strongly on its size, shape and surface chemistry, even when its overall chemical chemical composition is fixed.

For a sphere of radius $r$, the ratio of surface area to volume isHalving $r$ therefore doubles the surface area exposed per unit volume. At fixed total volume, making many smaller particles exposes more material to the surrounding medium. This helps explain why surface reactions and attached molecules can have a large effect on nanoparticles.^[ref]

The size of an individual nanoparticle also differs from the size of an particle aggregate containing many particles. Reports of particle size consequently need to identify what was measured. Gold nanoparticles provide a visible example: their interaction with light can differ markedly from bulk gold.

A particle aggregate is a cluster made from multiple primary particles. Its overall dimensions and internal arrangement differ from the size and shape of the individual nanoparticles. A measurement reporting particle size must consequently identify whether it resolves individual particles, a connected cluster or an effective size inferred from its motion.

Aggregation changes accessible surface area, fluid transport and interaction with light. A compact cluster and an open, branched cluster containing the same number of particles can therefore behave differently. Some measurement conventions reserve aggregate for strongly bonded or fused particles and use agglomerate for more weakly associated particles; experimental reports should state their convention and how sample preparation affects the clusters.