Reduction (adjective) refers to a kiln atmosphere which does not have enough oxygen in it to completely consume the fuel as it burns. Due to this deficiency, the flame pulls oxygen molecules out of the clay bodies and glazes, changing their character. Reduction can be also be used to describe clay bodies and glazes that are especially developed for reducing atmospheres.
Reduction (noun) refers to the state of being oxygen-starved. It can also be used to replace the full term, "reduction atmosphere".
Copper red glazes must be fired in a heavy reduction atmosphere, or they will come out green. (adjective)
This firing schedule calls for the kiln to go into reduction at cone 9. (noun)