In addition there is a property called emissivity (Greek epsilon) that determines how readily a hot object gives away heat by radiation.
I like to describe emissivity as the object's "darkness" at far infrared wavelengths around 10 microns. Why far infrared? That's where all objects anywhere near room temperature radiate. Far hotter objects like the sun radiate at shorter wavelengths in the visible and near infrared, with quite a bit of UV.
It's important to understand the difference because it's easy to get the misconception from the terms "absorptivity" and "emissivity" that these materials are somehow "diodes" for heat, that you can make a substance that absorbs all heat radiation inherently better than it radiates, or vice versa. The Second Law of Thermodynamics makes this impossible. Discrimination between incoming and outgoing radiation is possible for an object like a spacecraft in sunlight
only because it absorbs and radiates at different wavelengths. Objects can be bright at one wavelength and dark at another, but they can't be simultaneously dark and bright at the
same wavelength.
This has a very important practical implication for thermal engineering on the moon. Although the emissivity of an object, its "darkness" at far infrared wavelengths, controls how easily it radiates its own heat to deep space it also determines how easily that object
absorbs heat from other objects near its own temperature -- like the lunar surface. The moon is much cooler than the sun so it radiates far less heat per unit area, but the moon also appears much larger to you when you're standing on it. It occupies half of the entire sphere around you, so even its relatively weak radiation is very important. A radiator facing the lunar surface cannot cool below the temperature of that surface, so one designed to operate on the moon at noon when the surface exceeds 100C must be carefully shielded from as much of the surface as possible by reflectors.
It seems counterintuitive, but a radiator performs better with sunlight on it than when facing the warm lunar surface because the radiator surface can be designed to reflect the visible sunlight while still emitting efficiently in the far infrared, but it cannot be designed to emit in the far infrared at the same time it blocks those same far IR emissions from the lunar surface.
The moon is such a challenging place thermally that the Apollo spacecraft (including the PLSS) used a very brute force method to get rid of heat that was only practical because of the short duration of each mission: evaporating water into space. The CSM primarily used radiators that were kept facing space, but it too could evaporate water if needed for extra cooling when passing over the center of the day side of the moon.
