Longitudinal Chromatic Aberration
Colour fringing caused by different wavelengths focusing at different distances, producing purple in front of focus and green behind it.
In depth
Also called bokeh fringing, this error appears across the whole frame including the centre, and unlike its lateral cousin it cannot be fixed by rescaling channels — the misfocused colour simply is not sharp. It is worst in fast lenses shot wide open and diminishes markedly on stopping down two stops.
Examples in practice
A backlit portrait at f/1.4 shows magenta halos on the near edge of the hair and green on the far edge.