Õ: "ajahetk" (Google: üle 40000)
on russistlik tautoloogiline moodustis (nagu "õline õli"),
jäljendab момент времени eristamist näiteks
mitmesugustest jõumomentidest

olemus
(a) ajaskaala punkt
(b) määratlemata pikkusega väga lühike ajavahemik

Wiktionary:
Noun
instant (plural instants)
1. a very short period of time; a moment
She paused for only an instant, which was just enough time for John to change the subject.
2. a single, usually precise, point in time
The instant the alarm went off, he fled the building.
4. (law) present; current; extant
8. of the current month
Synonym: inst. (abbreviation)
I refer to your letter of the 16th instant in regard to traffic disruption.

ülevaateid
https://en.wikipedia.org/wiki/Instant

at that instant -- tol hetkel
at this instant -- sel hetkel
instantaneous -- hetk-; hetke-
instant month -- jooksev kuu
instant of failure -- tõrkehetk
instant status -- hetkeolek