Definition
The block header is the compact summary of a block that miners repeatedly hash when searching for a valid proof of work. It contains exactly six fields totaling 80 bytes: version (4 bytes), previous block hash (32 bytes), Merkle root (32 bytes), timestamp (4 bytes), bits/target (4 bytes), and nonce (4 bytes).
Miners hash this 80-byte header using double SHA-256. By changing the nonce (and sometimes the timestamp or Merkle root via the extra nonce), they produce different hash outputs until they find one below the difficulty target.
In Simple Terms
The 80-byte section of a block that miners hash. Contains version, previous hash, Merkle root, time, target, and nonce.
The 80-byte metadata section of a block containing the version, previous block hash, Merkle root, timestamp, difficulty target (bits), and nonce. This is what miners actually hash.