The classification of particles in a mixture by size based on their diameter or some other physical characteristic such as weight. The particle size distribution is the broad range of particle sizes in a mixture of particles, expressed as the quantity of the particles whose sizes fall between two measureable characteristics such as the weight percentage of the mixture between two defined diameters. The particle size distribution can also be expressed as a cumulative quantity such as the total weight expressed as a percentage of the mixture above a certain size.