D90 is particle size distribution D90, it represents the particle diameter corresponding to 90% cumulative (from 0 to 100%) undersize particle size distribution.
In other words, if particle size D90 is 117.8um, we can say 90% of the particles in the tested sample are smaller than 117.8 micrometer, or the percentage of particles smaller than 117.8 micrometer is 90%. D90 is a typical point in particle size distribution analysis.