iOS devices have a "lock screen" where to use the device you have to enter a passcode or password or fingerprint to gain access to the device. Usually if you pick up someone's phone this is the screen that you see.
Note the "slide to unlock" message at the bottom. If you asked most users what it means to unlock a phone, this is what they would think of, as they see it every day.
What possible citation could be better than the one provided? Apple has sold over 1 billion of these devices and they all share the same lock screen UI; 500,000 search results on Google pales in comparison as a citation.
Edit: I understand your point and agree that unlocking a device would not mean exactly the same thing to all people. I just don't agree that the parent needed an additional citation.
https://9to5mac.files.wordpress.com/2013/07/screen-shot-2013...
Note the "slide to unlock" message at the bottom. If you asked most users what it means to unlock a phone, this is what they would think of, as they see it every day.