The slow death of a blockchain caused by miners leaving en masse. Such a decrease in hash power can slow the chain down so much that the next mining difficulty adjust block-height is never reached and the blockchain effectively dies due to no confirmations.
The way many chains end up dieing. Death Spirals happen when a blockchain suddenly loses large amounts of hash power causing the amount of time it takes to find a new block to increase, which in turn raises the cost of mining, causing more miners to leave, raising mining costs more, making more miners leave... etc until eventually no more blocks are mined and the chain slows to a halt and dies.
Blockchains usually have rules in their difficulty adjustment algorithm to prevent this from happening.
* All terms and definitions may update as the Cryptionary improves.