Recently, there has been some discussion on establishing a definition for the term "natural user interface". My friend Richard Monson-Haefel (who just signed with O'Reilly on an iPad SDK book, congrats!) went through several iterations of a definition on his blog and ended up with this:
"A Natural User Interface is a human-computer interface that models interactions between people and the natural environment."
Wikipedia also has a paragraph describing natural user interfaces as invisible interfaces and lacking keyboard and mouse, but did not have a real concise definition. Ron George was a major contributor to the NUI wikipedia article. The first sentence says, in part, that a natural user interface is:
"...a user interface that is effectively invisible, or becomes invisible with successive learned interactions, to its users."
NUIGroup.com also has a wiki page with a lot of description language on the natural user interface, but no concise definition.
Multitouch on Windows. A key part of my approach is teaching the readers not just the APIs but also the new ways of thinking required for creating natural user interfaces. My first chapter is titled "The natural user interface revolution" (appropriate since it was also the title of my first blog post) and so right up front, I had to tackle the problem of defining natural user interface for my readers in a concise and comprehensive way.
I took into account both Richard's and the Wikipedia article's approaches, but I was not satisfied with what they had. I think Richard is on the right track, but the way he phrases it seems limiting. Whether or not he intended it this way, modeling the interactions between people and between people and the natural environment implies rather literal interface metaphors with NUI interactions that simulate real-world interactions, but there is no reason why this should be so. The Wikipedia's description talks about invisible interfaces, but to a lay-person this does not make sense and requires additional explanation of what an invisible interface means.
Now, I don't necessarily disagree with how Richard and the Wikipedia article are describing NUI. NUI does have something to do with how people interact with the environment, and NUI interfaces do seem to be invisible, but why are these descriptions true? To help figure this out, I turned to Bill Buxton's presentation in January where he talked about natural user interfaces. I took detailed notes and one particular thing that he said really resonated with me:
An interface is natural if it "exploits skills that we have acquired through a lifetime of living in the world."
I used that definition to write a section in chapter 1 on what "natural" means, and then derived my own definition. Below is an excerpt from chapter 1 of my book where I present my definition for natural user interface.
There are several different ways to define the natural user interface. The easiest way to understand the natural user interface is to compare it to other type of interfaces such as the graphical user interface (GUI) and the command line interface (CLI). In order to do that, let's reveal the definition of NUI that I like to use.Excepted from Multitouch on Windows by Joshua Blake
A natural user interface is a user interface designed to use natural human behaviors for interacting directly with content.There are three important things that this definition tells us about natural user interfaces.
NUIs are designed
First, this definition tells us that natural user interfaces are designed, which means they require forethought and specific planning efforts in advance. Special care is required to make sure NUI interactions are appropriate for the user, the content, and the context. Nothing about NUIs should be thrown together or assembled haphazardly. We should acknowledge the role that designers have to play in creating NUI style interactions and make sure that the design process is given just as much priority as development.
NUIs use natural human behaviors
Second, the phrase "designed to use natural human behaviors" tells us that the primary way humans interact with NUI is through our natural behaviors such as touching, gesturing, and talking, as well as other behaviors we have practiced for years and are innately skilled at. This is in contrast to GUI, which is described as using windows, menus, and icons for output and pointing device such as a mouse for input, or the CLI, which is described as having text output and text input using a keyboard.
At first glance, the primary difference between these definitions is the input modality -- keyboard verses mouse verses touch. There is another subtle yet important difference: CLI and GUI are defined explicitly in terms of the input device, while NUI is defined in terms of the interaction style. Any type of interface technology can be used with NUI as long as the style of interaction focuses on natural human behaviors.
NUIs have direct interaction with content
Finally, think again about GUI, which by definition uses windows, menus, and icons as the primary interface elements. In contrast, the phrase "interacting directly with content" tells us that the focus of the interactions is on the content and directly interacting with it. This doesn't mean that the interface cannot have controls such as buttons or checkboxes when necessary. It only means that the controls should be secondary to the content, and direct manipulation of the content should be the primary interaction method.
Chapter 1, "The natural user interface revolution"
I think this definition is very powerful. It gets right to the core of what makes natural user interfaces so natural in a way that does not restrict the definition to particular input technology or interaction pattern. It also can support the points-of-view presented by Richard and on Wikipedia, but in a more general way.
By talking about directly interacting with content, we establish that content interaction should be primary and artificial interface elements should be secondary and used only when necessary. This is an easier way to say the interface is invisible.
By framing the definition around natural human behaviors, we can talk about reusable patterns of behavior derived from human-human and human-environment interaction without implying we should model the interface after specific interactions. We can apply natural behaviors by reusing existing skills, which is what Bill Buxton was talking about. In the chapter, I spend a lot of time discussing these skills and how to apply them.
If you would like to read more on this, the entire chapter 1 is available for free download from Manning, where you can also pre-order the MEAP and read chapters as I write them.