What is the basic unit od?

Answer 1

OD often stands for "Optical Density". It has no true unit, as it is a ratio, although it is often labeled "AU", which I believe stands of Absorbance Units . An OD is "taken" at a particular wavelength to determine how much of that wavelength of light is absorbed and how much passes through, unaltered. For that reason it is often seen written in the following example-format: "OD630nm=1.023AU". This would mean that the Optical Density (or absorbance) of a sample, using light of the wavelength 630 nanometers, has a ratio of 1.023 to 1.

To simplify the answer a bit, it can be thought of as a liquid with more substance present being more dense, and therefore blocking more light from passing though it, than a solution with nothing added to it. This in turn allows one to visualize why a higher OD value is found when more material is present in a solution.

It is often used to read the results of chemical or biochemical assays such as an ELISA or a Protein Concentration Assay, although it's use is certainly not limited by such a short listing of tests and it is a very important & useful methodology in many technical fields.