Merkle Tree
A data structure where every leaf node contains a hash of a data block and every non-leaf node contains a hash of its child nodes, forming a tree that culminates in a single root hash (the Merkle root). Merkle trees allow efficient and secure verification that specific data is included in a large dataset without downloading the entire dataset — a property essential to blockchain scalability and light client operation.
“Bitcoin includes a Merkle root in each block header that summarizes all transactions in that block. A light wallet can verify a specific transaction is included by downloading only a small Merkle proof rather than the entire block.”
Hash
A fixed-length alphanumeric string produced by passing data through a cryptographic hash function (like SHA-256). Hashes are one-way functions — they can convert any input into a unique fingerprint, but the original data cannot be recovered from the hash. Hashing is fundamental to blockchain security, mining, address generation, and data integrity verification.
Block
A collection of transactions bundled together and permanently added to the blockchain. Each block contains a timestamp, transaction data, a reference (hash) to the previous block, and a nonce. This chaining of blocks through cryptographic hashes is what makes the blockchain tamper-resistant.
Node
A computer that maintains a copy of the blockchain and participates in validating and relaying transactions. Full nodes store and verify the complete blockchain history independently. Light nodes store only block headers for efficiency. Archive nodes retain every historical state of the blockchain. The more nodes a network has, the more decentralized and resilient it becomes.
Blockchain
A distributed, append-only digital ledger that records transactions across a network of computers in a way that makes it cryptographically secured and practically impossible to alter historical records. Each block contains transaction data and is linked to the previous block through a cryptographic hash, forming an immutable chain.