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A 32-byte hash derived from the bank's state at a given slot, included in every Solana transaction to prove the transaction was created recently and to prevent replay attacks. A blockhash remains valid for approximately 150 slots (~60–90 seconds at normal slot times); transactions submitted with an expired blockhash are rejected outright. Clients must fetch a fresh blockhash before signing and ideally reuse it for as short a window as possible to maximize landing probability.