Block Header
The block header is the compact summary at the top of a block that gets hashed to produce the block’s identity.
In depth
In Bitcoin it is exactly 80 bytes: version, previous block hash, Merkle root, timestamp, difficulty target and nonce. Miners change the nonce repeatedly and re-hash the header until the result meets the difficulty target. Because the header commits to every transaction via the Merkle root, hashing just the header effectively fingerprints the whole block.
Why it matters
It is the smallest piece of data that proves a block is valid — light clients sync using headers alone.