- ability to connect to the Internet (or other data network)
- supports user input and interaction
- offers multiple functionalities
- is lightweight and is less than 10"
Devices that are "mobile devices":
- smartphone (including "superphones", "world phones", and some feature phones)
- netbooks and ultraportable laptop
- personal digital assistant (e.g. iPod Touch)
- GPS navigation device (a.k.a. car or personal navigation device)
Wikipedia's definition is pretty broad. To them a mobile is "small, hand-held computing device, typically having a display screen with touch input and/or a miniature keyboard and less than 2 pounds (0.91 kg)". Wikipedia lists calculators, digital cameras, and MP3 players as mobile device. I normally love Wikipedia but I think they are stretching the term to entail pretty much any portable electronic device.
Perhaps these are all just types of handheld computing devices. I think my definition, however, fits the core functionality of what a device needs as a category term.
So although although all the devices mentioned so far have some computing power and many have network connectivity (as even many e-Readers and digital cameras now have). But an e-Reader and digital camera are pretty much single function devices. The Kindle e-Reader does have the cool ability to of user interaction in the ability to highlight passages of eBooks and share them online with others, but users can't create substantial content and it essentially it is a single function device (hence the name even).
Laptops may be portable but they aren't portable enough to allow ubiquitous access - a trait that I think is central to the concept of mobile device (opposed to just portable device).
This is the definition I'll be using for the indefinite future in my research. I'd love to hear any feedback on additional criteria to include. I'd also love to hear of any other devices that would fit this definition.