basically..hypochromicity is an effect showing by some compounds/substances (say DNA) a decreased absorbance of a wave length(chrome uses for colour,but) when it transformed physically (and in some extend chemically) to other state.

here, a sample of double stranded DNA absorbs less amount of wavelength (for instance a 260 nm ultraviolet) compared to its same quantity of single stranded DNA molecules..

This decreased absorbance in terms of dsDNA can be termed as "DNA Hypochromicity"