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I'm looking for a clear, single-sentence definition of usability.

A short definition of usability would make it much easier to convince people that usability is worth investing in. For example, a recent proposal for a Usability Roadshow to promote usability throughout the organisation was met with the obvious question: "OK, but what is usability exactly?"

Words like "effective", "efficient" and even "enjoyable" are often mentioned. But I still haven't seen a definition that I feel captures the essence of usability. I would like your help to come up with a short, standard definition to use next time somebody asks "What is usability?"

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I plan to write a final answer to this question soon, based on the answers below. Might do a little more digging around first. – Bennett McElwee Nov 29 at 20:39

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According to UsabilityNet, the ISO 9241-11 standard ("Guidance on Usability") defines usability as

The extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.

This nails down the context of use nicely, but it's a bit wordy and I would like to explicitly mention learnability.

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Yup - the ISO standard defines it pretty clearly. – Harry Brignull Nov 13 at 7:44
I think this definition might be good for professionals, but IMHO it's far to complicated to convince anybody to invest who doesn't know what usability is. – Phil Dec 9 at 6:39
I agree. My updated answer at uxexchange.com/questions/719/… is clearer. – Bennett McElwee Dec 12 at 0:35
This is by far the best definition of usability that we've got. It allows us to operationalise usability and generate specific measures around the notions of completion rate (effectiveness), time on task (efficiency) and surveys like SUS (satisfaction). When I use this definition with executives, I find they get it and see usability as something that can be really measured. So I disagree about it being too complicated. – David Travis Mar 3 at 20:58
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More ideas:

Usability means that the people who use the product can do so quickly and easily to accomplish their own tasks.
— Janice Redish and Joseph Dumas, A Practical Guide to Usability Testing


Captain Obvious style (nothing personal ;-)):

Usability can be thought of as how easy a product is to learn and how easy it is to use.
— Jeff Axup


And my favorite definition:

After all, usability really just means that making sure that something works well: that a person of average (or even below average) ability and experience can use the thing - whether it's a Web site, a fighter jet, or a revolving door - for its intended purpose without getting hopelessly frustrated.
— Steve Krug, Don't Make Me Think

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I think Axup's definition is the best of these. Redish/Dumas talk about the users' tasks, but I think usability is related to the product's intended tasks. If the user is using the wrong product, that's their fault, not the product designer's! And Krug sets the bar too low: the fact that I'm not hopelessly frustrated doesn't mean the thing is usable. – Bennett McElwee Nov 12 at 23:14
I agree that Redish/Dumas' definition focus on user rather then on product and needs some other words to make it real sense clear. As far as I understood the main words in Krug's definition are "can use". So we may shorten it like this: Usability means that a person can use the thing. (Of course because the thing is designed good.) – Kostya Nov 13 at 8:52
I actually agree with Krug here. The bar for usability is low. Did you open the door? Yes = Usable. Can you complete your task? yes = Usable. – Glen Lipka Nov 13 at 23:39
"Usable" isn't an all-or-nothing quality. If you can use something "without getting hopelessly frustrated" then that doesn't mean it's usable; it may just mean it's not hopelessly unusable. Anyway, I'm probably splitting hairs here. – Bennett McElwee Nov 17 at 9:59
I agree with you, Bennett. I like the beginning of Steve's definition, but the end sets the bar too low. Maybe we could find a better ending? What about "...without needing an inappropriate time to learn it."? Ok, that's not very elegant, but maybe the right direction...? – Phil Dec 9 at 6:44
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Some places I looked:

Hmm, how helpful are those? I think all of these dont help me because I set the bar too high. I don't want just usable. That is Don Norman 1.0 (The Design of Everyday Things). I want Don Norman 2.0 (Emotional Design).

Usability (to me) is when someone uses their product they turn around and say, "Dude, check this out, it's awesome!"

Not Usability is when someone uses your product and turns around and says "Dude, I can't figure this out." Or worse, they just shut it off.

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The trouble with this definition is that it doesn't help us build usable products. If none of the users say "awesome" then we know it's not usable; but how do we make it usable? That when we need a more objective definition. I like Nielsen's five points -- learnability, efficiency, memorability, errors, satisfaction -- and think they would make the good basis for a short definition. – Bennett McElwee Nov 12 at 23:16
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 That's what I was trying to get at between DOET and Emotional Design. When is Usable important as a goal? If something is Unusable, then clearly there is a problem. They can't use it. Usage FAIL. However, is being able to get your task complete always the goal? Sometimes, yes. The zipper on my jacket is usable. However, when I am building software, I want better than usable. I am really replacing Usable with Lovable. So you are right, it's not a definition of Usability. It's Lovability. The definition from Steve Krug below is a pretty good one for that. – Glen Lipka Nov 13 at 23:38
I love :) the idea of Lovability. You may be onto something! – Bennett McElwee Nov 17 at 10:00
To me, a user exclaiming that "It's awesome" is about user delight, not (or unlikely) about usability. – JeromeR♦ Dec 9 at 6:33
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This question really intrigued me. I was tempted to go and see how other people had defined it but then I thought, that might make me bias toward one definition over another. So, I decided to give it a shot. After quite a few trials I think I was able to boil it down to one sentence. However, in the end this is just the way I see it...

"Usability is the art of creating a positive cognitive response based upon a unique balance between information, design and interactivity."

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This quality may apply to usable products, but it seems to me that it could apply to other things too, for example works of art. I think it needs to be refined in order to really capture what usability is. – Bennett McElwee Nov 12 at 22:59
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Usability is a product's

  • learnability
  • effectiveness
  • efficiency, and
  • satisfaction

when it is used

  • by its intended users
  • to achieve its intended goals
  • in its intended context of use.

I think this may capture it. I based this on the ISO standard - I reworded it and added mention of learnability. I inserted bullets to reveal the structure, but it's really all one sentence, albeit wordy.

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