One OD260 (optical density) unit of DNA is the amount
of DNA that gives an absorbance reading of 1.0 at a wavelength of
260 nm for a sample dissolved in 1.0 ml total volume of
ddH2O, read in a 1 cm quartz cuvette. The OD value is
used to determine the concentration of an unknown oligo solution by
measuring the absorption at 260 nm.
1 OD260 corresponds to approx. 33 µg/ml of single
stranded DNA.