Refers to an event whereby a transaction has been processed by the network and is highly unlikely to be reversed. When a transaction is successfully entered into a block, it is said to be “confirmed”. This means that more than 51% of the Bitcoin nodes recognize that the transaction is valid and that there has been no attempt to double spend those bitcoins. Transactions receive a confirmation when they are included in a block. Although a single confirmation can be considered secure for low value transactions, it is advisable to wait for 6 confirmations or more. Each confirmation exponentially decreases the risk of a reversed transaction. 6 confirmations are generally enough to make the validy of a transaction beyond question. To protect against double spending, a transaction should not be considered as “confirmed” until a certain number of blocks in the blockchain verify the transaction. The classic Bitcoin client will show a transaction as “n/unconfirmed” until 6 blocks verify the transaction.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc