What exactly is an "electron-sponge"?

What exactly an "electron-sponge" [behavior/action/property/system] nickname is, and what makes a material an "electron-sponge" (preferably, quantitatively)?


From what I found, it's typically a molecular cluster or a metal complex which is capable of undergoing multiple oxidation and reduction waves, or is able of accepting several electrons without losing their chemical identity. However, this definition seems to be rather qualitative and quite vague. Also, it would be interesting to find out who and when coined this term.


