I would like to know an unambiguous term used to designate words that are spoken vanishingly rarely, if at all.
Words that are never spoken,
_________, illustrate the fundamental difference between spoken and written languages.
Such words as would be designated by the term might be scientific coinages, difficult to pronounce, prosodically awkward; whatever the reason, although the word can be pronounced, it is not; such a word is only encountered in written form.
When I attempted to recall the term, all I could come up with was
dictionary word, n.
(a) a word which may be found in a dictionary, a valid word; (b) a word which has been listed in a dictionary but which is rarely or never used, an obscure or recondite word.
["dictionary, n. and adj.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/52325?rskey=jj89IT&result=1&isAdvanced=true (accessed January 07, 2016).]
As can be seen, 'dictionary word', while it might be stretched to fit my use, is unsuitable in that (a) the first meaning given does not correspond to the meaning I intend, and (b) the second meaning given is inaccurate. The meaning in (b) is inaccurate and misleading because, for example, many scientific coinages are spoken rarely or never, but they will also never appear in a dictionary.
The term 'recondite word' might also be used, but it is inexact. 'Obscure word' suffers likewise.
Having failed to recall or discover the term, I am of course prepared to accept it does not exist; however, I seem to remember such a term being bandied about in some graduate English, Classics or Linguistics course in the early 1980s.