One that has succeeded, and more people know what it means than "High PPI". I'd say leap on it, and argue in court that people don't associate it with Apple, they associate it with not noticing pixels.
Which is even worse, because no other company can use this term that people "associate with not noticing pixels". Even when their devices outdo iPhone in terms of PPI, they will never be able to market it as retina.
If we really are going to use a term for "high PPI", it should be a standard word that nobody has trademarked.
If we really are going to use a term for "high PPI", it should be a standard word that nobody has trademarked.