In a recent discussion with a colleague, I mentioned "physical aging", and it quickly became apparent that the coworker thought they knew what I was talking about, but really didn't. "Physical aging" is one of those terms that is not unique enough to set itself off from everyday English and thus, in an (inadvertent ?) effort to avoid jargonese and provide clarity does quite the opposite and just causes confusion. It's similar with "Good Laboratory Practices", a term with a well-defined meaning courtesy of the US FDA, but if you ask chemist who has not worked for a pharma/medical device company "Do you follow good laboratory practices?", they will certainly say "yes!" with a good deal of pride and enthusiasm, despite their total ignorance of the documentation and process requirements.
So what is it? Physical aging occurs in polymers when they are below their glass transition temperature and are not at their equilibrium density. Despite the prohibition on flow that is implied by the glass transition, polymers will become denser over time leading to all sorts of unexpected results. Dr. Croll at North Dakota State (Fargo) has a number of publications on how this behavior can exasperate weathering and other degradation phenomenon.