Canonical terms. Every lesson uses these exact definitions.
| Term | Definition |
|---|---|
| Coin | A chain of digital signatures. Each owner transfers it by signing a hash of the previous transaction + the next owner's public key (§2). |
| Double-spend | Spending the same coin twice. The core problem; without a coordinator, the only defense is a single agreed-upon order of transactions (§1, §11). |
| Trusted third party | A mint/bank that orders transactions. Bitcoin's goal is to remove it. (§1) |
| Timestamp server | Conceptual ancestor of the chain: hashes a block of items + the previous timestamp, publishing the hash. Each timestamp reinforces the ones before it (§3). |
| Proof-of-work (PoW) | Find a nonce so that SHA-256(block header) < target (i.e. begins with enough zero bits). Costly to produce, trivial to verify (§4). |
| Nonce | The field in the header miners increment while searching for a valid hash (§4). |
| Target / bits | The threshold a hash must fall under. Adjusted (retargeted) to keep block production ≈ 10 min on average (§4). |
| Longest chain | The chain with the most cumulative proof-of-work (not most blocks). Treated as the truth; nodes always extend it (§4, §5). |
| Block header | 80 bytes: prev-hash, Merkle root, timestamp, target bits, nonce (§7). |
| Merkle tree | Binary hash tree over a block's transactions; only the Merkle root sits in the header. Lets old spent txns be pruned and enables SPV (§7). |
| SPV | Simplified Payment Verification: verify a payment with only block headers + a Merkle branch, without running a full node (§8). |
| Coinbase | The first transaction in a block; mints the block subsidy (initially 50 BTC) + fees to the miner. The minting + incentive mechanism (§6). |
| q / p | Attacker's vs honest network's share of hash power, p + q = 1. Security holds while p > q (§11). |
| z | Number of confirmations — blocks built atop the one containing your transaction. Catch-up probability falls ~exponentially in z (§11). |