A glidant is a substance that is added to a powder to improve its flowability. A glidant will only work at a certain range of concentrations. Above a certain concentration, the glidant will in fact function to inhibit flowability (Which means that there's a critical concentration to be used if increasing powder's flowability is intended with respect to the glidant and the powder properties). In tablet manufacture, glidants are usually added just prior to compression.
Examples of glidants include magnesium stearate, Aerosil (colloidal silicon dioxide), starch and talc.
A Glidant's effect is due to a counter-action to factors resulting in poor flowability of powders. For instance, correcting surface irregularity, reducing interparticular friction & decreasing surface charge. The result is a decrease in the angle of repose which is an indication of an enhanced powder's flowability.