XYZ colorspace is the foundation of tri-stimulus colorimetry.
CIE 1931 XYZ color space was created by the International
Commission on Illumination (CIE) in 1931, and provides a
numerical representation of human color vision response. The
important property of XYZ numbers is that two different light
spectra that have matching XYZ numbers will look to be the same,
when viewed under identical viewing conditions. Y corresponds to
the luminosity function, which corresponds to the human
spectral sensitivity of brightness.
When measuring emission (Luminance) or ambient (Illuminance)
light, these are in absolute units (candela per square meter, and
Lux respectively).
When measuring surface (reflective) or transmissive color, both
of which depend on an illuminating light source to have a color
appearance, it is conventional to assume the XYZ numbers are those
as if the illuminant was a standard one (such as 5000°K daylight -
D50 for graphic arts, D65 for paints, plastics, textiles, raw
inks), and the units are % relative to the perfect diffuser or
perfect transmitter.
All the other tri-stimulus colorspaces are based on
transformations of XYZ colorspace.