What is Jus Ad Rem?

Legal Definition
Jus ad rem is a Latin term of the civil law, meaning "a right to a thing:" that is, a right exercisable by one person over a particular article of property in virtue of a contract or obligation incurred by another person in respect to it and which is enforceable only against or through such other person. It is thus distinguished from jus in re which is a complete and absolute dominion over a thing available against all persons.

The disposition of contemporary civil law jurists is to use the term jus ad rem as descriptive of a right without possession, and jus in re as descriptive of a right accompanied by possession. Or, in a somewhat wider sense, the former denotes an inchoate or incomplete right to a thing; the latter, a complete and perfect right to a thing.
In canon law jus ad rem is a right to a thing. An inchoate and imperfect right, such as is gained by nomination and institution; as distinguished from jus in re, or complete and full right, such as is acquired by corporal possession.
-- Wikipedia
Legal Definition
Property, title. This phrase is applied to designate the right a man has in relation to a thing; it is not the right in the thing itself, but only against the person who has contracted to deliver it. It is a mere imperfect or inchoate right. 2 Bl. Com. 312 Poth. Dr. de Dom. de Propriete, ch. prel. n. 1. This phrase is nearly equivalent to chose in action. 2 Wooddes. Lect. 235. See, 2 P. Wms. 491; 1 Mason, 221 1 Story, Eq. Jur. 506; 2 Story, Eq. Jur. §1215; Story, Ag. §352; and Jus in re.
-- Bouviers Law Dictionary
Legal Definition
A term of the civil law, meaning "a right to a thing;" that fs, a right exercisable by one person over a particular article of property in virtue of a contract or obligation incurred by another person in respect to it, and which is enforceable only against or through such other person. It is thus distinguished from jus in re, which is a complete and absolute dominion over a thing available against all persons. The disposition of modem writers is to use the term "jus ad rem" as descriptive of a right without possession, and "jus in re" as descriptive of a right accompanied by possession. Or, in a somewhat wider sense, the former denotes an inchoate or incomplete right to a thing; the latter, a complete and perfect right to a thing. See The Carlos F. Roses, 177 U. S. 655, 20 Sup. Ct. 803, 44 L. Ed. 929; The Young Mechanic, 30 Fed. Cas. 873. In canon law. A right to a thing. An inchoate and imperfect right, such as is gained by nomination and institution; as distinguished from jus in re, or cemplete and full right, such as is acquired by corporal possession. 2 Bl. Comm. 312.
-- Black's Law Dictionary
Legal Definition
A right to a thing. See 3 Bl. Comm. 312.
-- Ballentine's Law Dictionary