Out-of-gamut Colour
A colour present in an image that the destination device or space cannot reproduce.
In depth
Out-of-gamut colours must be replaced with the nearest reproducible alternative, and how that substitution is made is the job of the rendering intent. The danger is not the substitution itself but the loss of separation: several distinct out-of-gamut colours can all map to the same in-gamut value, flattening detail into a solid block.
Examples in practice
Petals with subtle variation in a saturated orange flower print as a single flat colour once clipped to the printer's gamut.