In most other places in the world, the common terminology is mobile phone. But in the United States, this connected mobile device is known differently – as cellphone or cell phone.
But first, why is it known differently in the land of the free? Early mobile phone networks were built on the concept of cells, with an entire network made up of connected cells. As a layman, think of it like a honeycomb to get a good picture of it.

Each tower was the centre of each cell, and each cell was linked to adjacent ones, just like the cells in a honeycomb. And so, Americans began to call mobile devices cell phones.
But is it cellphone or cell phone?
The more common term in use is cell phone. This is how most Americans and Canadians write it. However, some have been known to write it as cellphone. I doubt that anyone gets marked down for writing it as either cellphone or cell phone.
For example, from time to time, I see articles in which the word smartphone is written as two words – smart phone. It irks me, to be honest. But I doubt that there is any committee of scholars giving out grades for how the word is written. I write it as smartphone all the time, because that is the widely accepted way to write it.
PS: A smartphone is a type of cell phone. A dumb phone is another type of cell phone. A feature phone is yet another type of cell phone. And of recent, a new category of cell phone has been introduced; a smart feature phone. An example is the Alcatel Go Flip 3, running KaiOS.
Similarly, the widely accepted word is cell phone (two words). I recommend that you write it that way, too. If you do choose instead to write it as cellphone, nobody is likely to wonder what you are referring to, I am sure.
Having said that, how do you write it – cellphone or cell phone? And if you are from somewhere else in the world where mobile devices have anotbet name, I’d like to hear from you. What is it called in your neck of the woods? Talk to me.
Way back from the days of EPOC, Symbian, Palm, PocketPC/Windows Mobile, BlackBerry/BB10, webOS, Windows Phone, Firefox, Ubuntu Touch, to Android, iOS, and KaiOS operating systems, founder of MobilityArena, Mister Mobility, has owned a few hundred smartphones and tablets, and counting.