Weird white box that won't go away

[kimulepolyglotte]

Hello everyone. So, today I was reading in iBook as usual, and clicked on words to define or highlight them and so on. But then all of a sudden, as I was clicking on one of them, this weird white box appears. And it won't go away! At least in iBook it won't. Can someone please tell me how to remove it?! And what is it anyways??

 

[hpulmer]

Reboot your device: first you double tap the home button and close the background apps by tapping and holding one until the red dot with the minus, then tapping each red dot. After that, keeping Sleep- and Home Button pressed until the iPad switches off and the apple appears again.