Block
A block is a bundle of confirmed transactions sealed with a fingerprint of the previous block.
Explained simply
Transactions are written to the blockchain not one by one but in bundles. Such a bundle is called a block. On Bitcoin one is created roughly every ten minutes, on other networks every few seconds.
Every block contains a fingerprint (hash) of the block before it. If someone altered an old block, the fingerprint would no longer match and all subsequent blocks would be invalid. That makes the chain tamper-proof.
From the familiar world
A bank statement where every page starts with the checksum of the previous page. If someone tears out a page or changes a number, the chain of checksums breaks from that point on.
An example
The very first Bitcoin block from 3 January 2009 (the "genesis block") contains a Times headline about bank bailouts as text. It is still included in every copy of the blockchain today.
Why it matters
The block principle explains why a blockchain cannot be manipulated after the fact. This is exactly the property that interests regulators, custodians and auditors.
To pass on
„A block is a page in the ledger. Every page carries the fingerprint of the previous one, so none can be swapped in secret.“
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