(n., adj.) a verbal aspect denoting a single, unrepeated action
When you cough or blink or knock on a door, it is implied by the word itself that you do so only once. The words we use to describe those actions—cough, blink, knock—can ultimately described as semelfactive.
Semelfactive is a term introduced to linguistics the mid nineteenth century. Etymologically, it derives from semel, a Latin word meaning ‘once’ (which is also the origin of words like semelparous and semelparity—the tendency of some creatures, like mayflies, to reproduce just once in their entire lifetime).
Technically speaking, semelfactive is not a ‘type’ of verb as such, but of verbal aspect. (Linguists, you might want to skip ahead a paragraph or two at this point—everyone else, brace yourself. Here comes the science part.)
Aspect is just a means of further classifying the tense of a verb. In English, for instance, we can talk about the past, present, or future tense of a verb. But each of these tenses can then be subdivided into different aspects—namely more nuanced forms that provide further information about the action that has been, will be, or is being carried out.
So the present tense can be further classified as the simple present (I talk); the present progressive (I am talking); the present perfect (I have talked); or the present perfect progressive (I have been talking). The same applies to the past and future tenses too, giving us aspects like the past perfect (I had talked), and the future progressive (I will be talking). And we can further complicate matters by adding the conditional aspect into the mix too, giving us the likes of the simple conditional aspect (I would eat) and the conditional perfect progressive aspect (I would have been eating).
All of these are all examples of so-called grammatical aspect—subtle changes to the meaning and implication of words and statements driven by changes borne out by their grammar. Semelfactivity, however, is something different. It is a form of lexical not grammatical aspect, meaning that what a semelfactive verb implies has less to do with (and is less often reflected by) the way in which it is used and manipulated grammatically, and more to do with what is unavoidably implied by its in-built, read-it-in-the-dictionary meaning.
We know by definition, for instance, that to cough is to perform a single self-contained act, with a definite beginning and end: a cough. There’s little we can do grammatically to that verb to change that—so when I cough or you cough or he coughed we each, again by definition, perform a cough. In order to change the semelfactivity of cough, we need to take a different approach—and attaching an expression of time is the perfect solution.
When a semelfactive verb is used alongside a temporal adverb or an expression of time, we force it to be reinterpreted as an iterative verb. In grammar, the iterative aspect is used to imply that an action—while still a single, self-contained act—has been repeated; the duration of the act itself has not changed, it’s just that it is being performed again and again. Take these two sentences, for example:
I coughed all night.
In the first sentence, we’re in the semelfactive aspect. I coughed just once, and that single act of coughing commenced and was all but instantaneously completed. By adding a timeframe to that sentence, however, we shift the verb cough into the iterative aspect. The single act of coughing itself hasn’t changed, because we can’t change the meaning of the word. Instead, it has been repeated, again and again—in this case, all night. If this timeframe were not to shift this sentence into the iterative aspect, we’d end up implying that that one, single, self-contained cough took the entire night to complete.
This shift from a self-contained act to a chain of repeated self-contained acts is characteristic of semelfactive verbs. Other verbs do not behave in this way: We talked all night, for instance, does not by definition imply that we spoke once, then stopped, then spoke again, then stopped, then spoke again, then stopped, again and again in a chain of individual self-contained moments of talking. Instead, We talked all night implies that the open-ended act of talking itself continued all night.
Talk, ultimately, is not a semelfactive verb, whereas cough—alongside the likes of knock, clap, punch, wink and blink—are.