Definitions of concentration:

noun: strengthening the concentration (as of a solute in a mixture) by removing extraneous material
noun: bringing together military forces
noun: the strength of a solution; number of molecules of a substance in a given volume (expressed as moles/cubic meter)
noun: the spatial property of being crowded together
noun: complete attention; intense mental effort
noun: increase in density
noun: great and constant diligence and attention