A measure of probability that a transaction could be rejected from the main chain. “ Zero confirmations ” means that transaction is unconfirmed (not yet in a block). “One confirmation” means that the transaction is included in the latest block in the main chain. “Two confirmations” means that the transaction is included in the block right before the latest one; and so on. The probability of a transaction being reversed is diminished exponentially as more blocks are added “on top” of it.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc