Leon says...

No, not really by definition, although its purpose is pretty much the same.
It reflects the heat rather than stops its movement into a cooler area.
Insulation pretty much absorbs heat.

Ah, takes me back to my college physics days. The reason why insulation
and a radiant barrier are needed is because heat can be transferred in
two ways: by physical contact of one substance that is hotter than the
one next to it, or by infrared radiation. Insulation protects against
the first type of heat transfer, but like you said, unless it
incorporates a reflective barrier, which a lot of it does now, it will
absorb infrared along with the rest of the house. This is bad in both
the summer and winter. In the summer, your house, including the
insulation, absorbs infrared from the sun. Then long after the sun goes
down, your house may still be hot and sticky because it has stored all
that energy and continues to radiate it onto you. It is bad in the
winter because when it absorbs the infrared, it eventually must radiate
it away, but the radiation goes in every direction. Some goes back into
your house (good), some is radiated away (bad). When IR hits a
reflective material, it bounces off instead of being absorbed. Infrared
isn't heat, it becomes heat when it is absorbed by some material.
Infrared barriers act the same way a mirror does, which is why they are
bright and shiny. Infrared barriers are good in summer and winter
because in the winter, it reflects the infrared back into your house
heating some object that can absorb it, and in the summer, it reflects
IR from the sun instead of allowing the material under it to absorb the
IR and beam it inside.

I know that's more than anybody wanted to know, but since I spent five
years cramming on this stuff, it should be useful for something.