Nomophobia is the irrational fear of being without your mobile phone or being unable to use your phone for some reason, such as the absence of a signal or running out of minutes or battery power.
A phobia is by definition an irrational fear. In the case of nomophobia, the events that the user fears are not terribly unlikely, so that part of it isn't irrational. What is irrational is the degree of discomfort the users feel at the thought of being, in effect, separated from their smartphones.
The word nomophobia is a portmanteau made up of no + mobile + phone + phobia. The term was created by YouGov, a research organization based in the United Kingdom. In a 2008 study, researchers reported that 53% of mobile users felt anxious when they were unable to use their mobile phones and over half of users never shut their phones off. Subsequent studies have found that the numbers have increased since then.