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Définitions from  ISO/TS80004-2 :2011 (EN)

Nanoparticle: nano-object with all external dimensions in the nanoscale where the lengthsof the longest and the shortest axes of the nano-object do not differ significantly.

Nano-object: discrete piece of material with one, two or three external dimensions in the nanoscale.

NOAA: Nano-Objects and their Aggregates and Agglomerates.

Définition recommended by the European Commission

The European Commission's definition, which distinguishes nanomaterials from non-nanomaterials, marks a first step toward developing nanomaterials legislation. In October 2011 the European Commission issued a recommendation that should serve as a frame of reference in determining whether or not a material should be considered a nanomaterial for regulatory and policy purposes:

According to the European Commission, a nanomaterial is "a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm-100 nm." 

The European Commission's recommended definition is merely a first step toward regulating nanomaterials and is not currently legally binding. Further discussion will be required to establish a definitive, broadly-accepted definition.

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