Hariri, a former prime minister for Lebanon, was killed in 2005. Hezbollah is widely suspected to have been behind the plot, though the Shiite resistance group claims to have evidence linking Israel to the bombing.
Appeals judges at the tribunal headquarters in the Netherlands agreed to use the country's definition of terrorism as an "act intended to spread terror" but also gave consideration to international characterizations of terrorism.
There are several definitions of terrorism used by international courts, though there is no universal definition of what constitutes an act of terror.
The court weighed differences between articles in its statute that recognize the authority of Lebanese law while at the same time relying on international norms of justice.
Presiding Judge Antonio Cassese in a statement said the idea of international terrorism was ripening but added that "the existence of a customary rule outlawing terrorism does not automatically mean that terrorism is a criminal offense under international law."
The court has a sealed indictment in the case and could issue arrest warrants for the accused when the tribunal's findings go public.