Glossary
Double-Spending
Spending the same coin twice — the original problem blockchains were invented to solve, prevented by consensus over a single ordering of transactions.
In a purely digital system, a unit of value is just bytes — bytes can be copied. Without a central ledger, what stops you from sending the same coin to two different recipients?
Blockchains solve this by making the network agree on one canonical ordering of transactions. Whichever spend confirms first wins; the other is invalid. Consensus is precisely the mechanism that makes double-spending detectable, and after enough confirmations, impossible to execute against any honest party.