Double-Spend
A double-spend is spending the same coins twice — the fundamental problem all consensus exists to prevent. The classic form: broadcast payment A to a merch
A double-spend is spending the same coins twice — the fundamental problem all consensus exists to prevent. The classic form: broadcast payment A to a merchant while secretly mining a longer chain containing conflicting payment B, then release it to erase A.
Why it matters
Every confirmation rule, every 51%-attack cost model, and every “wait for finality” warning traces back to this one attack. Small merchants accepting zero-confirmation payments get double-spent routinely; the entire architecture of confirmations, mempools, and reorg protection is the immune system built around it.