A subtle warm or cool bias within a colour that otherwise reads as fairly neutral — a 'warm white' vs a 'cool white', for example.
Colourwise estimates undertone from a colour's OKLCH hue angle: hues in the red-orange-yellow arc read as warm, the green-blue-purple arc reads as cool, with narrow transition bands treated as neutral.
Undertone is not the same as hue family — two colours can both be classified 'grey' (a neutral hue family) while having noticeably different warm or cool undertones.
Two greige paints can look almost identical in a tin but read distinctly warmer or cooler once on a large wall, because of undertone.
Undertone is one of the most common causes of a paint colour 'not working' with existing flooring, furniture or fittings.