A blockchain is a shared, append-only ledger maintained by a network of computers rather than a single central authority. Transactions are grouped into "blocks," each block is cryptographically linked to the one before it (forming a "chain"), and the network collectively agrees on which new blocks are valid through a consensus mechanism — most commonly Proof of Work or Proof of Stake.
What makes it different from a normal database
Three properties distinguish a blockchain from a conventional company-run database: decentralization (no single party controls it — thousands of independent computers, or "nodes," each hold a full copy), immutability (once a block is confirmed and buried under enough subsequent blocks, rewriting it becomes computationally infeasible), and transparency (most public blockchains let anyone inspect every transaction ever recorded, without needing permission). These properties are what let two strangers transact directly without a trusted intermediary like a bank verifying the exchange.
Why it matters for Shariah screening
The blockchain itself is infrastructure — a neutral record-keeping technology, comparable to how the internet or double-entry bookkeeping are neutral tools. It's not the blockchain that's assessed for Shariah compliance, but what's built with it: a specific coin's issuance model, a specific protocol's fee structure, a specific application's lending or gambling mechanics. This is the same tool-neutrality principle applied throughout our methodology — a technology isn't rendered impermissible because it can be used for haram purposes, only when a project's own design is built around one.