In microbiology, CFU stands for colony-forming unit, which is a unit used to estimate the number of viable microbial cells (bacteria, fungi, viruses, etc.) in a sample that are able to multiply via binary fission and form colonies on a solid medium. CFU is a measure of the number of living cells present in a sample, and it is determined by making serial dilutions of the sample and plating them on agar plates. The number of visible colonies present on the agar plate can be multiplied by the dilution factor to provide the CFU/mL value. CFU is a foundational concept in bacteriology, and isolated colonies on a plate are critical for all of the bacteriological analysis done in identifying and characterizing organisms.