An aliquot is a small volume of a stock solution. Generally speaking it is a non-specific amount, as indicated by the origin of the word. In Latin aliquot means some, several or a number of anything!
A typical use of aliquots in the lab is when you have frozen protein stock solution that you don't wish to keep thawing and freezing as repeated freeze/thaw cycles may cause the protein to denature. You would take small volumes of the solution and freeze. This would mean that you would only have to defrost a small volumes of the solution, and you would avoid repeated freeze thaw cycles.