DeFi stands for Decentralized Finance, meaning that crypto owners can recreate traditional financial instruments in a decentralized architecture. That is, outside the control of companies and governments.
In other words, DeFi can be defined as a set of new financial instruments based on decentralized systems and networks: it can be thought of as an excellent platform that currently uses mainly Ethereum blockchain and smart contracts.
The argument for decentralized finance (DeFi) has been made even more pressing by allegations of misuse of funds against a centrally controlled cryptocurrency.
Bitcoin and Ethereum are the original DeFi apps. Both are controlled by large networks of computers and not by central authorities.