General & Fundamentals

A cryptocurrency address is a unique alphanumeric string derived from a public key via cryptographic hashing, used to identify a destination or source for a blockchain transaction. Analogous to a bank account number, an address can be shared publicly to receive funds without exposing the corresponding private key. Address formats vary by network: Bitcoin uses Base58Check or Bech32 encoding; Ethereum uses a 42-character hexadecimal string prefixed with "0x".

An altcoin (portmanteau of "alternative coin") is any cryptocurrency other than Bitcoin. The term originated in the early Bitcoin community to describe coins launched on separate blockchains — such as Litecoin (2011) and Namecoin (2011) — that modified Bitcoin's source code. In contemporary usage, the term broadly encompasses all non-Bitcoin digital assets, including Ethereum, stablecoins, and tokens issued on smart contract platforms, though some analysts exclude Ethereum from the altcoin classification given its market dominance.

A block is the fundamental data unit of a blockchain, containing a batch of validated transactions, a reference to the preceding block (parent hash), a timestamp, a nonce (in Proof of Work systems), and a Merkle root summarizing all included transactions. Blocks are produced at regular intervals — approximately every 10 minutes for Bitcoin and 12 seconds for Ethereum — and are appended to the chain upon successful validation by the network's consensus mechanism.

A blockchain is a continuously growing, append-only ledger of records — called blocks — that are cryptographically linked and secured across a distributed, peer-to-peer network. Each block contains a cryptographic hash of the preceding block, a timestamp, and transaction data, forming an immutable chain. The structure ensures that any retroactive alteration of a block invalidates all subsequent blocks, providing tamper-evidence without reliance on a central authority.

A consensus mechanism is the set of rules and procedures by which nodes in a distributed blockchain network reach agreement on the valid state of the ledger — specifically, which transactions are legitimate and in what order they occurred. Consensus mechanisms solve the Byzantine Generals Problem: achieving reliable agreement among distributed participants who may be unreliable or adversarial. The two dominant paradigms are Proof of Work (PoW), which requires computational expenditure, and Proof of Stake (PoS), which requires economic collateral.

A cryptocurrency is a digital or virtual currency that uses cryptographic techniques to secure transactions, control the creation of new units, and verify the transfer of assets, operating on a decentralized network — typically a blockchain — without reliance on a central issuing authority such as a government or bank. The term encompasses both native network currencies (e.g., Bitcoin, Ether) and tokens issued on existing blockchain platforms. Cryptocurrencies may function as a medium of exchange, store of value, unit of account, or programmable money depending on their design.

Decentralization in the context of blockchain and cryptocurrency refers to the distribution of control, data, and decision-making authority across a network of independent participants rather than concentrating it in a single entity or small group. A fully decentralized network has no single point of failure, no central administrator, and no party capable of unilaterally altering the ledger or censoring transactions. Decentralization exists on a spectrum and encompasses three distinct dimensions: architectural (physical infrastructure), political (control over the system), and logical (data and interface structure).

Distributed Ledger Technology (DLT) is the broader category of database systems in which records are stored, shared, and synchronized across multiple nodes, locations, or institutions without a central administrator. Blockchain is the most prominent form of DLT, but the category also includes directed acyclic graph (DAG) structures (e.g., IOTA's Tangle, Hedera Hashgraph) and other consensus-based distributed databases. DLT is distinguished from traditional distributed databases by its use of cryptographic proofs and consensus mechanisms to ensure data integrity without requiring trust in any single participant.

A cryptocurrency exchange is a platform that facilitates the buying, selling, and trading of digital assets. Exchanges are broadly categorized as centralized exchanges (CEX) — operated by a company that holds custody of user funds and maintains an internal order book — or decentralized exchanges (DEX) — non-custodial protocols that execute trades via smart contracts directly between users' wallets. Centralized exchanges typically offer higher liquidity, fiat on-ramps, and advanced order types, while decentralized exchanges offer self-custody and permissionless access.

Fiat currency is government-issued money that is not backed by a physical commodity such as gold or silver, but derives its value from government decree (Latin: fiat — "let it be done") and the trust and authority of the issuing state. All major national currencies — including the U.S. Dollar (USD), Euro (EUR), and Japanese Yen (JPY) — are fiat currencies. In the cryptocurrency context, fiat is used as the baseline reference for pricing, on-ramp/off-ramp transactions, and regulatory classification of stablecoins.

A fork in blockchain terminology refers to a divergence in the protocol rules governing a network, resulting in two or more versions of the blockchain. A soft fork is a backward-compatible upgrade in which nodes running the old software can still validate blocks produced under the new rules. A hard fork is a non-backward-compatible change that creates a permanent split in the chain, resulting in two distinct networks with a shared transaction history up to the fork point. Hard forks may be contentious (resulting in competing chains) or planned (coordinated upgrades).

The Genesis Block is the first block of a blockchain, hardcoded into the protocol software rather than mined through the standard consensus process. It has no parent block hash (or references a null hash) and establishes the initial state of the ledger. Bitcoin's Genesis Block (Block 0) was mined by Satoshi Nakamoto on January 3, 2009, and contains the famous coinbase message: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks." The 50 BTC block reward from the Genesis Block is unspendable due to a quirk in the original Bitcoin software.

A cryptographic hash function is a deterministic mathematical algorithm that maps an input of arbitrary size to a fixed-length output (the "hash" or "digest") in a manner that is computationally infeasible to reverse. Hash functions used in blockchain systems must satisfy four properties: determinism (same input always produces same output), pre-image resistance (cannot derive input from output), collision resistance (infeasible to find two inputs producing the same output), and avalanche effect (small input changes produce drastically different outputs). Bitcoin uses SHA-256; Ethereum uses Keccak-256.

Immutability in blockchain systems refers to the property whereby data recorded on the ledger cannot be altered, deleted, or tampered with after it has been confirmed and buried under sufficient subsequent blocks. This property emerges from the cryptographic chaining of blocks: altering any historical block would require recomputing the proof of work (or equivalent) for that block and all subsequent blocks, and outpacing the honest network — a computationally infeasible task on sufficiently large networks. Immutability is a foundational guarantee of blockchain's trustless value proposition.

A ledger is a record-keeping system that tracks the ownership and transfer of assets. In traditional finance, ledgers are maintained by trusted intermediaries (banks, clearinghouses). In blockchain systems, the ledger is distributed across all network nodes and maintained collectively through consensus, eliminating the need for a trusted central record-keeper. Blockchain ledgers may use a UTXO model (Unspent Transaction Output, used by Bitcoin) or an account/balance model (used by Ethereum), each with distinct implications for privacy, scalability, and smart contract design.

The mempool (memory pool) is the holding area maintained by each full node where valid but unconfirmed transactions await inclusion in a block. When a user broadcasts a transaction, it propagates across the peer-to-peer network and enters the mempools of nodes that receive it. Miners and validators select transactions from the mempool — typically prioritizing those with higher fee rates — and include them in the next block they produce. During periods of high network congestion, the mempool grows and transaction fees rise as users compete for limited block space.

A node is any computer that participates in a blockchain network by maintaining a copy of the ledger and communicating with other participants. Full nodes download and independently verify the entire blockchain history, enforcing all protocol rules without trusting any third party. Light nodes (SPV clients) download only block headers and rely on full nodes for transaction verification. Mining/validator nodes additionally participate in block production. The number and geographic distribution of full nodes is a primary measure of a network's decentralization and censorship resistance.

A peer-to-peer (P2P) network is a distributed architecture in which participants (peers) communicate and share resources directly with one another without routing through a central server or intermediary. In blockchain systems, P2P networking enables nodes to broadcast transactions and blocks, discover new peers, and synchronize ledger state without any central coordination point. Bitcoin's P2P network uses the Bitcoin protocol over TCP/IP, with nodes connecting to a set of peers and relaying valid transactions and blocks they receive.

A private key is a secret cryptographic value — typically a 256-bit random number — that grants its holder the exclusive ability to authorize transactions from the corresponding blockchain address. The private key is used to generate a digital signature that proves ownership of funds without revealing the key itself. The relationship between private key and address is one-way: the address can be derived from the private key (via the public key), but the private key cannot be derived from the address. Possession of a private key constitutes de facto ownership of the associated funds — "not your keys, not your coins."

A public key is the publicly shareable component of an asymmetric cryptographic key pair, mathematically derived from the private key via elliptic curve multiplication (Bitcoin and Ethereum use the secp256k1 curve). The public key can be freely distributed and is used by others to verify digital signatures produced by the corresponding private key, confirming that a transaction was authorized by the key's owner. The public key is further hashed to produce the blockchain address. The mathematical relationship between private and public key is one-way: deriving the private key from the public key is computationally infeasible.