Serial dilution is a laboratory technique used to create a series of solutions with decreasing concentrations of a substance. This is achieved by taking a known volume of a concentrated solution and mixing it with a specific volume of a diluent, such as water. The process is repeated multiple times, resulting in a range of diluted solutions that can be used for various experiments. This method is commonly used in microbiology to estimate the number of bacteria or viruses in a sample. By diluting the sample, researchers can count the colonies that grow on a culture plate, allowing them to determine the original concentration of the microorganisms present.