The "elution time" of a solute is the time between the start of the separation (the time at which the solute enters the column) and the time at which the solute elutes.  In the same way, the elution volume is the volume of eluent required to cause elution.  Under standard conditions for a known mix of solutes in a certain technique, the elution volume may be enough information to identify solutes.  For instance, a mixture of amino acids may be separated by ion-exchange chromatography.  Under a particular set of conditions, the amino acids will elute in the same order and at the same elution volume.