Definition of Forum rei
This term may mean either (1) the forum of the defendant, that is, of his residence or domicile; or (2) the forum of the res or thing in controversy, that is, of the place where the property is situated. The ambiguity springs from the fact that rei may be the genitive of either reus or res.
That's the definition of Forum rei in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.