What is a smart contract? That depends on the model of computation we are talking about. There are two competing computational models used in decen- tralised databases: the virtual computer model and the UTXO model. The virtual computer model is used by Ethereum 5 . It models the database as the in-memory state of a global computer with a single thread of execution deter- mined by the block chain. In the UTXO model, as used in Bitcoin, the database is a set of immutable rows keyed by (hash:output index). Transactions de- fine outputs that append new rows and inputs which consume existing rows. The term “smart contract” has a different meaning in each model. A deeper discussion of the tradeoffs and terminology in the different approaches can be found in the Corda introductory paper