[eointierney]

Terms of art are pre-scientific jargon that survive mainly due to utility. They are common in crafts such as pottery or the Law. They are references to heuristically derived conveniences. They're linguistically cool.

In Math, lemma is a term of the art.

[mehwoot]

They are references to heuristically derived conveniences

That's every word in existence

[eointierney]

Yes, and this is not trite. Every word we hang on to has special meaning. Think about the amount of words we've forgotten.

Could a computational linguist chime in please? How many words in how many human languages will never be spoken again in 2021?

[bradrn]

Not a computational linguist (or a linguist at all), but this is an area I know a bit about. The question you ask is at the centre of the study of ‘glottochronology’ [0]:

> The original method presumed that the core vocabulary of a language is replaced at a constant (or constant average) rate across all languages and cultures …

Unfortunately, the only major discovery glottochronology has revealed is that rates of change vary too much to be of any use:

> in Bergsland & Vogt (1962), the authors make an impressive demonstration, on the basis of actual language data verifiable by extralinguistic sources, that the "rate of change" for Icelandic constituted around 4% per millennium, but for closely connected Riksmal (Literary Norwegian), it would amount to as much as 20%.

And there are other factors affecting replacement rate as well. For instance, a curious trait about the non-Austronesian languages of New Guinea is that the word ‘louse’ is practically never replaced: it evolves according to normal sound change, but never gets thrown out entirely. By contrast, many languages have a taboo against mentioning the names of the deceased; this can speed up lexical replacement if those people have names homophonous with common words.

For these reasons, I suspect that a general answer to your question will be extremely difficult — if not impossible — to find.

[0] https://en.wikipedia.org/wiki/Glottochronology

[glup]

In linguistics, lemma is also a term of art.