A blockchain transaction that has been broadcast to the network and appears in the mempool but has not yet been included in a confirmed block. Accepting zero-confirmation transactions involves risk because they can theoretically be double-spent before being mined into a block. Some merchants and payment processors accept zero-confirmation transactions for low-value purchases due to the speed advantage, implementing additional fraud detection measures to mitigate the double-spend risk.
Example
“The coffee shop accepted the Bitcoin payment as a zero confirmation transaction since the $5 purchase amount was too small to warrant waiting for block confirmation.”