Sorry, but that link is to the National Association of Home Builders. Again, there is no statutory definition of "principal residence". If you believe there is, please post the statute. I will then admit my error.

The definition of principal residence is identical to the one used to determine whether you may qualify for the $250,000 / $500,000 capital gain tax exclusion for principal residences.
This is true only because the section of the tax code dealing with the capital gain exclusion also uses the term "principal residence" without a statutory definition.

The IRS issued guidance in 2002 to help taxpayers determine their principal residence. This guidance can be reviewed at http://www.irs.gov/newsroom/article/...105042,00.html.

For taxpayers with multiple homes, the regulations list several factors relevant to determining which home is the principal residence. Among these are amount of time used; place of employment; where other family members live; the address used for tax returns, driver’s license, car and voter registration, bills and correspondence; and the location of the taxpayer’s banks, religious organizations or recreational clubs.
I consider the IRS to be an authoritative source for this kind of information. There are a number of complex factors that are taken into consideration in determining principal residence. In my experience, the amount of time lived in a residence is the most important factor but certainly not the only one. The individual's present location is not of much relevance.