Definition
A blockchain is a shared ledger that records transactions in linked blocks copied across many computers. Each block contains a cryptographic fingerprint of the one before it, so changing old records is detectable.
How it works
Participants broadcast transactions, and a consensus method such as proof-of-work or proof-of-stake decides which new block is accepted. Because each block carries the previous block's hash, altering one record would force every later block to be recomputed and accepted by the network.
A transaction is usually considered more final as more blocks are added after it, called confirmations. Blockchains differ in speed, fees and access: public networks allow anyone to participate, while private ones limit membership. The ledger proves what was recorded, not that the real-world claim behind an entry is true.
Formula / convention
Confirmations = 1 + number of blocks added after the block that contains the transaction
Illustrative example
Illustrative: a network targets one block every 10 minutes, so it produces about 6 blocks per hour and 6 × 24 = 144 blocks per day. A payment included in the newest block has 1 confirmation, and after 5 more blocks it has 6, about 5 × 10 = 50 minutes later on average.
What to watch for
Blockchain records are hard to alter but not error-proof: software bugs, weak governance and concentrated control can still cause failures, and mistakes on-chain are generally irreversible. Being recorded on a blockchain does not make an asset valuable, safe or legally enforceable.