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Solidity is the DEV-created (i.e. Ethereum Foundation-created), Javascript-inspired statically-typed language that can be used to create smart contracts on the Ethereum blockchain. There are other languages you can use as well (LLL, Serpent, etc). The main points in favour of Solidity is that it is statically typed and offers many advanced features like inheritance, libraries, complex user-defined types and a bytecode optimizer.
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