A Stokes number (Stk or St) of a particle sitting in a fluid is a characterization of its tendency to flow with the fluid. It is the ratio of two characteristic times:
Stk = t0 u0 / l0
(The latter two divide out to be the reciprocal of a time.) Particles with a low Stokes number tend to "stay with" the fluid flow, whereas with a higher stokes number take time to respond to the fluid motion, presumably due to inertia, i.e., a more massive particle.