An Ethereum smart contract is a program stored at an address on the Ethereum blockchain. It contains code and data, and the Ethereum Virtual Machine (EVM) runs its code when an interaction requests it. A contract is not inherently a legal agreement or a self-starting robot: execution follows programmed rules when triggered, and changing Ethereum’s state normally requires a transaction and gas.
What “smart contract” means on Ethereum
Ethereum.org defines a smart contract as “a program that runs on the Ethereum blockchain” and describes it as code (functions) plus data (state) at a specific blockchain address. In practical terms, the code specifies what the program can do, while its state holds information that can change as the program is used. Ethereum’s introduction to smart contracts explains the basic model.
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The word “contract” here refers to software. It does not, by itself, mean that the code is a legally binding agreement between people or organizations; that is a separate question from how the program works.
How a contract is created and run
1. Developers write and compile the program
Developers commonly write Ethereum contracts in languages such as Solidity or Vyper. The source code is compiled into bytecode that the EVM can execute. Ethereum’s smart-contract language guide describes the languages and compilation step.
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2. A deployment transaction creates a contract account
Deploying a contract is a transaction that creates a contract account and places its executable code in Ethereum’s state. The EVM is the execution environment shared by Ethereum nodes; it processes instructions according to deterministic rules so nodes can reach the same result. See Ethereum’s technical introduction and its EVM documentation.
3. An interaction requests a function
A user or application can request a contract function through an interaction. If the requested operation changes state—such as updating a balance or recording a new value—it normally needs a transaction that is included in a block. The contract executes the relevant code, and the resulting state change is recorded on Ethereum. Contract execution is therefore triggered by an interaction or another supported execution mechanism; the contract does not independently notice events in the world and act on them.
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A vending machine is a useful, limited analogy: when it receives a valid input and selection, it follows its programmed logic to produce an output. The analogy illustrates predictable input and output, not independent awareness or activity.
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Ethereum has two broad account types. An externally owned account (EOA) is controlled by a private key and can initiate transactions. A contract account has code and is controlled by that code rather than by its own private key. A contract can be called by an EOA, an application, or another contract; what happens depends on its programmed rules. Ethereum’s account documentation explains the distinction.
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Many familiar interactions still involve a person’s wallet submitting a transaction. That is not the only possible arrangement: account-abstraction designs can allow a smart-contract account to initiate a write, and a paymaster may cover the gas. The exact flow depends on the application and account setup. Ethereum’s interaction guide describes these variations.
Transactions, gas, and read-only calls
Gas measures the computational work required for Ethereum operations. Deploying a contract and making a state-changing call consume gas, because they require computation and change blockchain state. A read-only call that does not change state does not cost gas under Ethereum’s interaction guide. Read-only access may still be mediated by an application or service, but it does not make a state change on-chain.
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In the common wallet flow, a transaction is signed and submitted for inclusion in a block, with gas paid as part of its execution. Account abstraction can alter who initiates the write and who covers its gas, so an EOA directly paying every contract call is not a universal rule.
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They follow code; they do not guarantee correct outcomes
Execution is deterministic: given the same relevant state and inputs, the EVM follows the same rules. That predictability does not make a contract safe or sensible. A bug, exploit, or poorly designed rule can also be executed as written. Ethereum’s dapp guidance emphasizes careful design and testing.
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They cannot fetch off-chain facts on their own
A contract cannot directly retrieve information from an ordinary website, a sensor, or another off-chain source. If a program needs external data—for example, a reported exchange rate—it relies on an oracle or another mechanism to make that information available on-chain. The contract can then process the supplied data according to its code.
They are public and can be composed
Ethereum contracts can generally be inspected and called by other contracts or applications, subject to the design of the particular system. This composability lets applications build on existing on-chain programs, but it also means an application’s behavior can depend on other contracts and their state.
Deployed code is generally fixed, but applications can use upgrades
Once deployed, a contract’s code is generally immutable by default, and blockchain interactions are not simply reversible. Some applications use upgrade patterns that separate the contract users interact with from the contract containing business logic. The intermediary can be configured to route calls to different logic, changing what users reach without rewriting the original deployed code. Whether an application can be upgraded—and who has authority to do so—depends on its specific design. Ethereum’s upgrade guide covers these patterns.
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Why the definition matters
Calling something an Ethereum smart contract tells you that executable code and data are associated with an on-chain contract account, and that the EVM can run the code when an interaction requests it. It does not tell you whether the program is secure, whether its rules are fair, whether it can be upgraded, or whether it forms a legal agreement. Those questions require looking at the particular code, application design, and any separate legal terms.
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