thermal conductivity
Thermal conductivity is a single number that tells you how good a material is at conducting heat. It is why a metal saucepan handle burns your hand but a plastic or wooden one stays cool, even when both have been on the stove: metal has a high thermal conductivity, plastic and wood have a low one.
It is the property k that appears in the conduction rate law Q/t = k A ΔT / L: for a given area, thickness, and temperature difference, a bigger k means faster heat flow. Its SI units are watts per metre per kelvin, W/(m K). Copper sits around 400 W/(m K) and other metals are similarly high, while still air is about 0.024 and foams and wool are only slightly higher, which is exactly what makes them good insulators.
Because trapped air conducts so poorly, most everyday insulation works by holding many tiny pockets of still air, in a duvet, a foam cup, or an animal's fur. Note that thermal conductivity is a property of the material itself; how well a whole object resists heat flow also depends on its thickness and area, which is a related but separate idea (sometimes packaged as thermal resistance or an R-value).
Copper's thermal conductivity (about 400 W/(m K)) is more than ten thousand times that of still air (about 0.024), which is why a thin metal sheet feels cold to the touch while a thick foam pad feels warm.
High k means a good conductor; very low k, like trapped air, means a good insulator.
Thermal conductivity is a property of the material alone; a whole object's resistance to heat flow also depends on its thickness and area, a related but distinct quantity.