Blockchain Technology — Term Reference

A distributed ledger where records (transactions) are grouped into blocks, each cryptographically linked to the previous block via its hash. The chain structure makes historical records tamper-evident — changing any block invalidates all subsequent blocks. No single party controls or maintains the ledger.

A record of transactions replicated and synchronized across many independent nodes (computers) simultaneously. No central server holds the authoritative copy — all nodes hold equal copies and continuously verify consistency. The distribution eliminates single points of failure and control.

A container in the blockchain holding a batch of transactions, a timestamp, and the hash of the previous block. The hash of the previous block is what creates the "chain" — each block is cryptographically linked to all preceding blocks, making the record tamper-evident.

The first block in a blockchain, with no previous block to reference. All subsequent blocks trace their cryptographic chain back to the genesis block. It is the foundation of the chain's immutability.

The technique of including the previous block's hash within each new block. This links every block to all preceding blocks. If any historical block is modified, its hash changes, which invalidates every block that follows — the tamper is immediately detectable by comparing block hashes across the network.

A participant in the blockchain network that holds a copy of the ledger and participates in transaction validation. Full nodes store the complete blockchain history. The distributed nature of nodes means the ledger has no single point of failure — the network continues even if many nodes go offline.

The protocol by which all nodes in a blockchain network agree on which blocks are valid and should be added to the chain. Without a central authority, consensus mechanisms ensure all honest nodes agree on the same version of the ledger. Common mechanisms: Proof of Work (PoW) and Proof of Stake (PoS).

A consensus mechanism where nodes (miners) compete to solve a computationally expensive puzzle (finding a nonce that produces a hash meeting a difficulty target). The winner adds the next block and earns a reward. Computationally costly to produce but easy to verify. Makes forging history prohibitively expensive — requires outcomputing the entire network.

A consensus mechanism where validators are selected to create blocks in proportion to the amount of cryptocurrency they have staked (locked up as collateral). Dishonest validators lose their stake — economic punishment replaces computational difficulty. More energy-efficient than Proof of Work.

An attack where a single entity gains control of more than 50% of a blockchain network's consensus power (mining power in PoW, staked value in PoS). This majority allows them to override consensus: approve fraudulent transactions, double-spend, or rewrite recent history. Practically infeasible against large networks due to cost.

The property that once data is recorded on a blockchain and confirmed by the network, it cannot be altered without detection. Immutability comes from hash chaining (modifying history breaks all subsequent blocks) and distributed consensus (an attacker must control the majority of the network to override the legitimate history).

A self-executing program stored on a blockchain that automatically enforces the terms of an agreement when specified conditions are met. No intermediary needed — the code executes automatically and transparently. Examples: automatic insurance payouts on flight delays, automatic property transfers upon payment confirmation.

A blockchain open to anyone — any participant can join the network, validate transactions, and view the complete ledger. Bitcoin and Ethereum are public blockchains. Maximally decentralized; anyone can audit the full history. Suitable for trustless environments where participants don't know each other.

A blockchain where participation is restricted to known, authorized parties. Used in enterprise and government settings where full public transparency is not desired but tamper-resistance and distributed trust are still valuable. Examples: supply chain management consortiums, interbank settlements.

In Proof of Work mining, a number that miners vary when computing a block's hash. Miners increment the nonce repeatedly until they find a hash that meets the difficulty target (e.g., starts with a certain number of zeros). Finding the right nonce requires massive computation but verifying it requires a single hash computation.

An attack where the same digital currency is spent twice — possible if an attacker can rewrite transaction history. A 51% attack could enable double spending. Consensus mechanisms and hash chaining make double spending effectively impossible on established blockchains — a transaction confirmed by multiple subsequent blocks is extremely difficult to reverse.