I recently read about different cultural changes and how they're named. Let's simply say stone age, bronze age or space age or internet age. In all of these "ages", there's something (a discovery or an invention or an idea) that just, kind of, "sticks out".
What's the word for this thing? (And if so, what might it be today that the civilizations after us call our time?)
Both 'epoch-making' and 'epochal' constitute proper usage
The synonyms, epoch-making and epochal, are equally acceptable. Deciding when to use one word over the other will depend on the context. For example, if one wants to emphasize a development that defines an era, then epoch-making will probably convey that meaning more readily to a general audience than epochal, due to the two-part, hyphenated word construction.
Thus, if an individual reads:
"Routine production of stone tools was an epoch-making advancement for hominins."
She might think to herself:
"Ah, regular production of stone tools made the paleolithic epoch."
Both words, epoch-making and epochal, came into use during the mid-1800s.1 The OED notes that epoch-making was "orig. said chiefly of scientific discoveries or treatises; now extended to designate any remarkable or sensational event, publication, etc."2
I provide details below on the lesser-known word, epochal.
Detailed information regarding 'epochal'
Oxford English Dictionary
Of the nature of an epoch; forming an epoch; epoch-making.3
Frequency (in current use): Frequency Band 4
Etymology (epoch): < late Latin epocha, < Greek ἐποχή stoppage, station, position (of a planet), fixed point of time, < ἐπέχειν to arrest, stop, take up a position, < ἐπί + ἔχειν to hold. Compare French époque, Italian epoca.
Examples of earliest use
1857 M. Pattison in Westm. Rev. Oct. 389 The..epochal crises of affairs.
1866 W. R. Alger Solitudes Nature & Man ii. 80 [David Hume's] place in the history of philosophy is of epochal importance.
1877 J. W. Dawson Origin of World vi. 127 Warring..has suggested that the Mosaic days are epochal days.
Oxford Dictionaries - English
epochal | ADJECTIVE
Forming or characterizing an epoch; epoch-making.4
‘the epochal scale of change in the East’
‘the beginning of Jesus's human life is an epochal event’
‘The current information revolution can be termed as the fifth epochal event since the birth of the human species.’
epochal - 1. highly significant or important; especially bringing about or marking the beginning of a new development or era; "epochal decisions made by Roosevelt and Churchill"; "an epoch-making discovery"
significant, important - important in effect or meaning; "a significant change in tax laws"; "a significant change in the Constitution"; "a significant contribution"; "significant details"; "statistically significant5