I see it used a lot, but haven't seen a definition that makes complete sense.
Wiktionary says "characterized by an adequate or excellent level of performance or efficiency", which isn't much help.
Initially I though performant just meant "fast", but others seem to think it's also about stability, code quality, memory use/footprint, or some combination of all those.
I think this is a "real" question - but if enough people reckon this is a subjective question, that's an answer in itself.
In university, performant means that you have an optimal solution in O(n) notation. You might get a remark on your paper about using made up words instead of the proper terminology though.
In business, performant means that your customer hasn't complained about the speed (yet). Your product is also buzzword compliant.
The lay-person's definition is simple: "something that's performant performs well; performant == high performance, !performant == low performance"
I don't see it being applied to measures other than performance. Ugly, hackish, and unstable code can be performant.
What does “performant” software actually mean?
Nothing. Everything. Whatever you want. Whatever the other guy in the discussion wants (which is usually the opposite of yours).
I [...] haven't seen a definition that makes complete sense.
Exactly.
It's a great ignition source for flamewars, that's about all it's useful for.
Exception: if you have a precise definition and everybody agrees on that definition and that definition is in place before the argument starts, then and only then can you have a meaningful discussion / argument.
[BTW: it's the same for "scalable" and "strongly typed", among others.]
Performant is a word that was made up by software developers to describe software that performs well, in whatever way you want to define performance.
It is a word that is not in the dictionary yet, but I think it should be. It's a good word, one that makes sense, based on a different form of an existing word, and one that actually has a valuable purpose.
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Contrast that with the phrase, "begging the question," which has been used incorrectly for so long that it may actually be recognized soon in its incorrect form as common usage "correct" english.
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Having a performant product means that you have successfully become buzzword-compliant. Further, you may now consider that product to be approved by Marketing Weasels.