Immutability

Immutability is one of the main properties of blockchain. It enables a blockchain ledger to remain unchanged and unaltered. This means that once you have agreed on a transaction and recorded it, it can never be changed. 

Smart contract

A smart contract is an irreversible self-executing contract between two or more parties. Essentially, it is a computer code that underlines the conditions and terms of agreement of the contract and is publicly available on the blockchain network. Smart contracts allow different parties who often don't trust each other to transact without the need for a central authority, legal system or any other type of intermediary.