Where does ‘Liquid‘ come from?

Devices such as the iPhone XR and 11 and iPad Pro 3rd and 4th generation all support the Liquid Retina display. Why is the word ‘Liquid’ in the name?, I often assume the reason for ‘Liquid’ being in the name is to do with the fact that the iPad Pro supports ProMotion technology which gives the display a 60 to 120Hz refresh rate. If that’s the case I’ve basically answered my own question, but for example if you look at the iPhone 11 that has a Liquid Retina display but doesn’t support ProMotion at least according to iPhone 11 Tech Specs, so where does ‘Liquid’ come from?.