DeFi stands for decentralized finance. DeFi is an abbreviation of the phrase decentralized finance which generally refers to the digital assets and financial smart contracts, protocols, and decentralized applications (DApps) built on Ethereum.
To get a feel for the products and services that exist in the Ethereum DeFi ecosystem, you can check out our DeFi rankings which tracks the current value locked into popular DeFi smart contracts. DeFi as a concept is to be able to offer almost all the financial services available with legacy and centralized institutions, typically banks, but on the blockchain. Whatever traditional services financial institutions provide can be expected to be offered through DeFi. In short, Defi is blockchain-based financial services that traditional financial services map into, creating new services or derivatives stemming from blockchain's unique features.
Although we have had concepts like distributed computing and decentralized computing for decades, DeFi is in fact a new concept or an expansion of the application scope of blockchain as infrastructure.
The vast majority of these applications are currently created on Ethereum, but in principle, other platforms (like TRON) with smart contract capabilities could work too