(n.) a verbal aspect used in some languages to indicate single or short-lived actions
We could write an entire book on things other languages do that the English language doesn’t (hey, that’s an idea...) and the so-called momentane verb aspect is yet another.
Some languages—English very much not included—are able to form or inflect their verbs to indicate that an action being described was only brief, momentary, or occurred just one time, and the momentane aspect allows them to do precisely that.
The momentane is not a particularly common grammatical feature, and it appears that only a handful of major languages—like Finnish and Estonian, as well as some Native American languages, like Navajo—make much use of it.
On Twitter, we used a Finnish example in the verb käydä, meaning to visit—which can be inflected to form the momentane derivative käväistä, meaning to drop in on someone only very briefly. Likewise, kääntää is a Finnnish verb meaning to turn, while its derivative käännähtää means to turn sharply, quickly, or just once. Järkkyä is to shake or tremble, but järkähtää is to momentarily shock or surprise someone, or to knock them senseless. And while kiekua is to crow like a rooster, keikaista is to cock-a-doodle-do just once, like a cock crowing at dawn.
In grammar, the concept of the momentane aspect is similar to (and at times overlaps with) two other verbal aspects, namely the iterative and the semelfactive. In fact, so closely allied are these terms that some linguists don’t consider them separate concepts at all, and often use them synonymously.
In contexts where these terms are considered individually, the iterative aspect is typically reserved for brief events that occurred repeatedly over a short period of time (like someone knocking on a door, or banging a drum), while the semelfactive is often used of single, short, one-time events that were entirely completed in a single moment (like a cough, a blink, or a wave of a hand). In that sense, these aspects contrast with others like the frequentative (which describes isolated events that happen on numerous occasions over time), and the habitual (which, as its name suggests, refers to actions performed usually, ordinarily, or customarily).