What is Res Inter Alios Acta?

Legal Definition
Res inter alios acta, aliis nec nocet nec prodest (Latin for "a thing done between others does not harm or benefit others") is a law doctrine which holds that a contract cannot adversely affect the rights of one who is not a party to the contract.

"Res inter alios" has a common meaning: "A matter between others is not our business."
-- Wikipedia
Legal Definition
Evidence. This is a technical phrase which signifies acts of others, or transactions between others.

2. Neither the declarations nor any other acts of those who are mere stran-gers, or, as it is usually termed, any res inter alios ada, are admissible in evidence against any one when the party against whom such acts are offered in evidence, was privy to the act, the objection ceases; it is no longer res inter alios. 1 Stark Ev. 52; 3 Id 1300.
-- Bouviers Law Dictionary
Legal Definition
A thing done between others, or between third parties or strangers. See Chicago, etc., R. Co. v. Schmitz, 211 111.446, 71 N. E. 1050.
-- Black's Law Dictionary
Legal Definition
The acts and declarations either of strangers, or of one of the parties to the action in his dealings with strangers. See 211 111. 446, 71 N. E. 1050.
-- Ballentine's Law Dictionary