Celadon is traditionally a high fire (2350F+ 1240C+) feldspathinc glaze that is colored with iron oxide, fired in reduction, and which is anywhere from a pale ice blue to a deep jade gray-green. There are apocrophal anecdotes to the derivation of the name.... and I've seen a ton of these supposed stories..... but no one apparently knows that for sure.
Celadon certainly looks great on stoneware, but it is clearly at its best when fired onto a white colored or porcelain body.
NOTE: It is possible with modern ceramics technology to produce the celadon looking color and surfaces at lower firing temperatures and even without reduction firing using commercial glaze stain colorants. Lacking technical testing equipment most people would have a hard time telling these pieces from ones fired in the traditional manner.
Is this really "celadon". How many angels can dance on the head of a pin?
It is not celadon in my book.... but if the color and look is all that is important... who is to really say.