A stock solution is a concentrated solution that will be diluted to some lower concentration for actual use. Stock solutions are used to save preparation time, conserve materials, reduce storage space, and improve the accuracy with which working lower concentration solutions are prepared.
In chemistry, a stock solution is a large volume of common reagent, such as hydrochloric acid or sodium hydroxide, at a standardized concentration. This term is commonly used in analytical chemistry for procedures such as titrations, where it is important that exact concentrations of solutions are used. Stock solutions do not necessarily come in concentrations of simple numbers; for example a solution could be 0.1 M HCl.
In biochemistry, the term is often used to refer to a concentrated solutions, from which one can dilute into a working concentration of solution.