Achromatic is an adjective that means “free of color.”
In printing, achromatic is used as a synonym for “black and white.” A black and white print has shades of grey, but greyscale is also considered to be achromatic because it lacks hue, which means it cannot be classified as a subset of the colors red, green, blue (RGB) or yellow.
In optics, an achromatic lens is a composite lens made of up at least two different types of glass. The lens is designed to reduce color errors. An achromatic lens transmits light without decomposing it spectrally.
The Intranet Encyclopedia of Science has more information about achromatic lenses.