A hostile takeover of a cryptocurrency by acquiring at least 51% of the mining power associated with it. When a single entity, person or group, controls more than half of the computing power (hash rate) of a cryptocurrency network, it can issue conflicting transactions to harm the network.
It is not tampering with the content of a specific block. It consists of building separately a competing version of the chain which they have picked up from the official network. The development of such alternative chain happens within an environment which is disconnected from the official network.
The attackers have more power than the original network and therefore can develop their own alternate chain faster than the original. At a certain point, they reconnect to the network and start to synchronize. The longer chain (the alternate one) wins in the confrontation and is adopted by the rest of the network.
All transactions that had been recorded in the official chain after the branch are eliminated and replaced by the blocks of the alternate chain. In this way double-spending becomes possible.