The programming language most associated with Ethereum smart contracts is Solidity.
Solidity is a statically typed, contract-oriented language designed for programs that run on the Ethereum Virtual Machine, or EVM. Developers write contract logic in Solidity, compile it into bytecode, and deploy that bytecode to Ethereum, where network nodes execute it according to the chain’s rules.
The language was proposed in August 2014 by Gavin Wood and later developed by a Solidity team led by Christian Reitwiessner, with contributions from other Ethereum developers. Its syntax was influenced by languages including JavaScript, C++, and Python, which helped make it familiar to many programmers.
Solidity is not Ethereum’s only smart-contract language. Vyper also targets the EVM and emphasizes a more restricted, security-focused design; Serpent and Mutan were earlier alternatives. Rust is widely used in other blockchain ecosystems, but it is not the standard language normally meant by this Ethereum question.