A modern depiction of a resonator or cavity resonator, a body with a hole in it, colored black on the inside, that absorbs radiation when cold and emits radiation when hot. [1]
In radiation thermodynamics, a resonator is a sphere, typically made of iron, with a hole in it, the cavity of the body colored black, whereby through the hole it can absorbs electromagnetic radiation when cold, acting like a perfect black body, or, conversely, emits electromagnetic radiation, out of the hole, when hot. [1]

The term “resonator” seems to have been introduced by German physicist Max Planck in 1895 during his work on the study of the absorption and emission of radiation by an electrically charged system having the same eigenfrequency as the radiation; the system, it seems, being the “resonator”, as he called it. [2]

In a modern sense, the terms resonator, cavity resonator, and black body, seem to the nearly synonymous; although this needs to be fact checked.

It could be the case that the term "black body" is used when the resonator is cold and acts as a perfect absorber. It also could be the case that Planck's usage of the term "resonator" has something to do with the system being electrically charged with a certain frequency.

1. Gribben, John. (2002). Quantum Physics: a Beginner’s Guide to the Subatomic World (pg. 13). DK Publishing.
2. Mehra, Jagdish and Rechenberg, Helmut. (2000). The Historical Development of Quantum Theory (resonator, 27+ pgs; “resonator” coined by Planck, pg. 34). Springer.

Further reading
‚óŹ Bryan, G.H. (1906). “The Entropy of Radiation” (resonator, pgs. iv), Nature, 74.

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