P =

Here's an interesting definition of "portability", a concept with a lot of similarities with "mobile". It refers to products, only products.
P = "T" (usage time of a single charge) by "U" (ubiquity of usage: 1 only indoor 5 indoor and outdoor) divided by "w" (the weight of the product in grams).
According to this index, the most portable product, among a 17 range products considered, has been a Bluetooth earphone, with a 4,6 index. At the other extreme you can find the 17.inch laptop, with a 7,500 times lower index (0.0006045).
More details about this model and more things about portability in "Portable Consumer Electronics", by S. Canumalla and P. Viswanadham.

WHAT'S MOBILE?

It is more than a family of technologies that can be used in mobility and it's more than a bunch of activities made ​​using these technologies.
It's a state of mind, which determines attitudes and social behavior, which determine the adoption of some technological equipments. A "brainframe" - in other words.
"Mobile" is an experience that gradually, at different rates depending on the individual, is more and more intertwining with the lives of people, changing not only behaviors but also how they look at life.
If previously the dimension of mobility was severely limited, today it enjoys the same freedom of all other social dimensions, freed from technological constraints. Can be 'filled' with exactly the same things and the same intentions of virtual life in the broadest sense.