OD600 is an abbreviation indicating the absorbance, or optical density, of a sample measured at a wavelength of 600 nm. Also seen written as "O.D. 600," "o.d. 600," "OD600" etc. It is a common method for estimating the concentration of bacterial or other cells in a liquid.
Measuring the concentration can indicate the stage of cultured cell population, i.e. whether it is in lag phase, exponential phase, or stationary phase. The final concentration reached is dependent on the culturing medium.
OD600 is preferable to UV spectroscopy when measuring the growth over time of a cell population because at this wavelength, the cells will not be killed as they would under too much UV light. UV light has also been shown to cause small to medium-sized mutations in bacteria, potentially altering or destroying genes of interest.