Definition:

Unit that is used in microbiology to estimate the number of viable bacteria or fungi in a sample

A colony is a cluster of bacteria growing together. To measure the CFU, bacterial cultures are added to agar plates, often by serially diluting the original sample as it might be too concentrated to count. The number of visible colonies (CFU) present on an agar plate can be multiplied by the dilution factor to provide the CFU/ml value e.g., 1 x 106 CFU/ml.