Dr Philipp Paech covers “What is a Smart Contract” in the University of Oxford’s “Law and Autonomous Systems Series".
“The term ‘smart contract’ refers to computer code that is designed automatically to execute contractual duties upon the occurrence of a trigger event. Referring to a 1997 paper by Szabo, the simple example of a vending machine is often cited to explain the concept: upon insertion of a specific type of coin, the computer programme instructs the machine to release the good. Yet, there are different ways to understand the term ‘smart contract’.”
You can read the article in full here.