This really depends on what you're trying to define, permit or prohibit, and why.
For example, the law in the UK relating to using a mobile phone whilst driving is:
- a hand-held mobile telephone, or
- a hand-held device (other than a two-way radio) which performs an interactive communication function by transmitting and receiving data.
if the driver is in such a position as to be able to see, directly or by reflection, a television screen or similar apparatus except one showing information:
- about the state of the vehicle or its equipment e.g. screen warning lights;
- about the location of the vehicle and the road on which it is located e.g. some GPS tracking devices;
- to assist the driver to see the road adjacent to the vehicle e.g. reversing cameras; or
- to assist the driver to reach their destination e.g. sat navs .
That page also notes that situations such as wearable devices or using a phone to scan/pay for goods are unclear and would have to be decided by the courts. They were not envisaged when the primary legislation was enacted in 1988.
The actual wording of the legislation is
using a hand-held mobile telephone or other hand-held interactive
41D, Road Traffic Act 1988 and
A device referred to in paragraphs (1)(b), (2)(b) and (3)(b) is a
device, other than a two-way radio, which performs an interactive
communication function by transmitting and receiving data.
(a)a mobile telephone or other device is to be treated as hand-held if
it is, or must be, held at some point during the course of making or
receiving a call or performing any other interactive communication
(c)“interactive communication function” includes the following:
(i)sending or receiving oral or written messages; (ii)sending or
receiving facsimile documents; (iii)sending or receiving still or
moving images; and (iv)providing access to the internet;
The Road Vehicles (Construction and Use) (Amendment) (No. 4) Regulations 2003
This is why judges are sometimes reported as asking odd questions in court such as "what is a BlackBerry?". The answer in this case would be "an interactive communication device" and it would therefore be established that a BlackBerry would be a mobile phone within the meaning of the Act. A BlackBerry might not be a "mobile phone" within the meaning of different legislation, such as trade tariffs where it might be classed as a computer or a PDA or something different.
If you're trying to prevent people using devices for information security reasons your definition is likely to be quite different from a definition stopping people phoning/texting while driving.