It's turned into a generic term that usually means the ability to remove all corporate branding from the UI. A lot of it is in the ability to apply themes. For example the Icon style for Android 6 (Marshmallow) is now considered retro.

https://en.wikipedia.org/wiki/Android_Marshmallow#/media/Fil...

I particularly despise the way Samsung wants to use impose a "bulging squircle" icon shape and I always find a way to override that (Nova Launcher still works well for this use case).