51% Attack
A 51% attack is when a single party controls the majority of a network’s mining power or stake and can rewrite recent history.
In depth
With majority control, an attacker can mine a private chain, spend coins on the public one, then release the longer private chain to erase that spend — a double spend. They still cannot forge signatures or steal others’ coins directly. The attack is prohibitively expensive on large networks but has hit several smaller ones.
Why it matters
It defines the outer limit of blockchain security and why network size matters.