Originally from the phrase 'mobile device', since the mid-2010s the word device has come more and more to mean an electronic device with a GUI and an internet connection, including those that are not mobile. This is part of a semantic shift where 'computer' has come to mean 'computer with a full keyboard', leaving a whole class of things that are technically computers but we don't call computers; along with this has come the common-usage assumption that the key function of a computer is to access the internet.
A device may be a desktop computer, a laptop, an iPad or other tablet, a Smartphone, a netbook or Chromebook, and often a Kindle or other eReader.
This is an interesting area of terminology, as the terms 'computer', 'device', and 'phone' are all used in older, formalized forms, but now appear in common usage as new words with new meanings. This is frequently acknowledged in passing, but mostly for older folks; for example, Google Help has a section entitled "See devices that have used your account", which goes on to specify that this includes "computers, phones, and other devices". For those of us old enough to remember when phones and computers were completely different things this sentence is a slightly surreal attempt to manage terms changing so quickly that their etymology is both confusingly twisted and completely obvious to most native English speakers.