
Simultaneity
We encountered this new form of a known word, today.
Generally, one sees the noun form of the oft-used adjective, simultaneous, expressed as "simultaneousness," though this is not universally accepted as a correct construction.
Simultaneity is a synonym for it, expressing the quality of things occurring at the same time.
Anyone else have any favorite examples of the curious situation created when one form of a word is common, but another version/form of it is rare?