A defined way of turning numbers into colours, so that the same numbers mean the same colour wherever they are used.
A colour model (such as RGB) plus the specifics that pin it down: the primaries, the white point and the transfer function. sRGB, Display P3 and Adobe RGB are all RGB models but different colour spaces.
'RGB' is not a colour space on its own. R 255, G 0, B 0 is a different red in sRGB and in Display P3.
CSS assumes HEX values are sRGB; color(display-p3 1 0 0) names a redder red.
Colour values are only portable when the space travels with them, which is what embedded profiles and CSS colour functions are for.