Orisi is a distributed system which allows for more secure smart contracts to be created by relying on multiple oracles to detect changes outside of the block chain. Checking external state is already possible with the standard Bitcoin system, but it relies on only one outside contact, an oracle, to check the validity of the given requirements.
To mitigate the reliance on one possibly vulnerable outside source, Orisi consists of a distributed network of oracles. For a contract to go through, the majority of oracles have to decide that the given conditions have been met.
Using outside sources to validate conditions can lead to a myriad of different uses, including [two-tiered arbitration].