Achromatic waveplate (AWP) is a pair of crystal quartz and magnesium fluoride plates.
Because of difference in dispersion in these uniaxial positive crystals it is possible to calculate thickness of each plate so that birefringent phase shift in assembly changes very slowly over certain wavelength range.
This is a way to obtain achromatic zero order waveplates.
Such AWPs are necessary for various devices and may replace a number of ordinary quartz waveplates operating at a single wavelength only.
Elan makes a series of achromatic waveplates with phase shifts shown in graphs below.