Sealevel is Solana's parallel transaction processing runtime. Unlike most blockchains that process transactions sequentially, Sealevel can execute thousands of smart contracts simultaneously — as long as they do not read or write the same accounts.

Sealevel is able to parallelise execution because Solana transactions must declare all accounts they will read and write upfront. This declaration allows the runtime to identify which transactions are independent of each other and can safely execute at the same time.

Sealevel is a major contributor to Solana's throughput advantage. Rather than being limited by single-threaded execution, Solana can leverage modern multi-core processors to execute independent transactions in parallel.