Smart Contract, also called as self-executing contract, blockchain contract or digital contract, is a set of instructions that automatically performs the obligations under the agreement, which parties have committed to. By being immutable and cryptographically secure, Smart Contract gives us the security of Blockchain Technology, that is, it provides distributed trustworthy calculations.
Where ever there is a mention of Blockchain on the internet, you will always find reference to Smart Contracts too. The word Smart Contract originated in 1997, when Nick Szabo, used it to describe physical objects that change their behavior based on data. Today, in Blockchain, we used this word in a different sense. It is used to describe a computation that happens on the Blockchain which is influenced/triggered by external events/information such as the weather.