Nonce

Since hash functions are deterministic, hashing the same block header twice gives the same result. The nonce is the field miners change between attempts: each new value yields a completely different, unpredictable hash. Mining is simply iterating nonces (billions of times per second on modern hardware) until one hash meets the difficulty target.

Bitcoin's 32-bit nonce space is too small for modern ASICs, which exhaust it in a fraction of a second — so miners also vary the timestamp and the coinbase transaction (the "extra nonce") to expand the search space.