Similar term(s): particle size.
Airborne particles have irregular shapes, and their aerodynamic behaviour is expressed in terms of the diameter of an idealised spherical particle known as aerodynamic diameter.
Particles are sampled and described on the basis of their aerodynamic diameter, which is usually simply referred to as particle size.
Particles having the same aerodynamic diameter may have different dimensions and shapes.
Source: based on the GreenFacts Digest on Air Pollution
For more on the aerodynamic diameter of particles, see the Module 3 of the US EPA
Aerodynamic(s) - Coarse particles - Fine particles - Particulate matter