Interesting.
Is there a definition of a “tort in violation of the law of nations.”?
Not in the 1789 law, which is why SCOTUS has struggled to define it in recent years. (This is not the first Alien Tort Act claim to reach SCOTUS in the last few years.)
Congress in 1789 was probably thinking about piracy-- if a Spanish pirate robbed a French ship, and took the loot to the U.S., the French shipowner could sue the pirate here.
In recent years, there have been attempts to bring suits under the Act over torture, genocide, terrorism and other things said to violate contemporary understandings of international law. The Court today basically kicked the issue back to Congress and told them to define the law better.