transaction

A transaction is the basic unit of action on a blockchain — a signed instruction to change the shared ledger, most commonly to move value from one party to another. It is the equivalent of a single entry in an account book or a single signed cheque: it says who is sending what, to whom, and proves the sender truly authorized it. Everything a blockchain records is ultimately built out of transactions; blocks are simply convenient bundles of them.

What makes a transaction trustworthy is the digital signature attached to it. The sender signs the instruction with a private key that only they hold, producing a signature that anyone can check against their public key but no one can forge. The network verifies three things before accepting it: that the signature is valid, that the sender actually controls the funds being spent, and that those funds have not already been spent elsewhere. Only then does the transaction become eligible to be written into a block.

Once a transaction is included in a block and that block is buried under enough later blocks, it is settled — effectively permanent and visible to all. There is no central clearing house, no business hours, and no way for the sender to quietly reverse it afterward. This is the quiet power of the model: a final, self-verifying record of who agreed to do what, produced without anyone needing to trust the people on the other side.