Double-Spending
Spending the same digital money twice — the problem that made digital cash impossible until Bitcoin solved it.
Double-spending is using the same funds in two conflicting transactions — trivial with copyable digital files, catastrophic for money. Before Bitcoin, every digital payment system needed a trusted central ledger-keeper to prevent it. Bitcoin’s breakthrough (2008) was solving double-spending with no central party: a public blockchain ordered by Proof of Work makes rewriting spent transactions economically prohibitive.
In practice, protection grows with confirmations — each block buried on top of a transaction multiplies the cost of reversing it. That’s why exchanges credit large deposits only after several confirmations, and why a 51% attack is the theoretical (and on small chains, occasionally real) route to double-spends.