What is Smart contract?

A smart contract is a program that runs on a blockchain, executing automatically when its predefined conditions are met. Once deployed, the contract’s code is publicly visible on the blockchain and cannot be altered. The output is guaranteed by the same consensus mechanism that secures the blockchain itself.

The term was coined by computer scientist Nick Szabo in the 1990s, but the concept became practical with the launch of Ethereum in 2015, which was designed from the start to host smart contracts as a first-class feature.

What smart contracts do

Smart contracts power a huge range of on-chain applications:

• Tokens — virtually every cryptocurrency other than Bitcoin, including stablecoins like USDC, runs as a smart contract on an existing blockchain rather than on its own.
• Decentralized exchanges — protocols like Uniswap let users swap tokens directly from their wallets, with no intermediary holding custody.
• Lending protocols — users supply assets to a smart contract and earn yield, or borrow against collateral without a credit check.
• NFTs — non-fungible tokens are smart contracts that track unique ownership of digital items.
• Decentralized identity and reputation systems.

Why this matters for payments

Most crypto payments that look like “simple coin transfers” under the hood are actually smart contract calls. When you pay with USDC, you are calling the USDC smart contract’s transfer function. When you swap one token for another on a decentralized exchange, you are calling a smart contract that executes the trade atomically.

For merchants, the practical implication is that smart contracts introduce a class of risk that simple Bitcoin payments do not: contract bugs. A vulnerability in a token’s smart contract can lead to loss of funds. Reputable tokens (USDC, USDT, DAI) are extensively audited and have been operating safely for years, but lesser-known tokens carry meaningful risk.

Audits and verification

Before interacting with a smart contract, look for evidence of a security audit by a reputable firm (OpenZeppelin, Trail of Bits, Spearbit, Certora). For payments, sticking to well-known, audited tokens on mainstream networks is the safest approach.