Apochromatic Lens
A lens corrected to bring three wavelengths of light to a common focus, rather than the two of a conventional achromat.
In depth
An achromat aligns red and blue but leaves green focusing slightly differently, producing a residual colour error. An apochromat closes that gap, which shows up as cleaner edges on high-contrast subjects and the absence of colour fringing in out-of-focus areas. The label is unregulated, so manufacturers apply it loosely.
Examples in practice
Apochromatic correction is why a good macro lens renders a black beetle's carapace without magenta halos on the highlights.