How much light a surface reflects like a mirror rather than scattering it, from flat matt to high gloss.
Measured in gloss units with a glossmeter at a set angle (commonly 20°, 60° or 85°, chosen by how glossy the surface is), relative to a polished black glass reference. Higher gloss concentrates surface reflection into the highlight, so the colour seen away from it looks deeper and more saturated.
Gloss changes how a colour looks without changing its pigment, which is why one colour in two sheens can fail to match.
The same paint in eggshell and in matt reads as two slightly different colours on adjoining walls.
Colour specifications that omit gloss level leave room for a visible mismatch even when the pigment is identical.