www.alphadictionary.com

"Epsilontist" is a word used by Paul Halmos in his essay "How to write mathematics". It appears in this sentence:

Symbolic presentation, in the sense of either the modem logician
or the classical epsilontist, is something that machines can write and few
but machines can read.

The essay can be found online at https://faculty.washington.edu/heagerty ... oWrite.pdf and appears on page 142. But I've never seen the word elsewhere, and an online query doesn't find anything at all.

Is anything known about its origin and established definition?

I managed to come up with two places that speak of epsilontics, so I guess that's what an epsilontist would engage in. Here they are:

https://www.wordsense.eu/epsilontic/
https://en.wiktionary.org/wiki/epsilontics

If anyone cares to share just what those definitions might mean in plain English, I'd appreciate it. I'm no theoretical quantum mathematician.

Here's what etymonline has to say about epsilon itself.

Thank you, Slava! I'm pretty familiar with the terminology of mathematics (having worked for a mathematical publisher my entire working life), but the meanings of most terms are beyond my understanding. To come across a mathematical term that I'd never seen before was rather unsettling. At least now I can file it in its proper niche.