According to the PCGS standards, PR70 and MS70 mean no defects under 5X magnification. I don't know what NGC's standards are, but presumably they're not all that different.

It used to be a contentious question whether any coin should be graded PR70 or MS70. That grade, according to the more conservative view, was reserved for a coin entirely free of defects, something which exists only in theory. While the debate may continue as an academic matter, for practical purposes it's moot, since all the grading services now apply MS70 and PR70 to coins in the real world, all of which at close enough scrutiny are less than perfect. But the notion of perfection is still loosely associated with the 70 grade, so it fairly goes without saying that any coin with a defect visible to the naked eye isn't close to a 70.