Alternative phrasing for an unconfirmed transaction.
In cryptography, a zero-knowledge proof enables one party to provide evidence that a transaction or event happened without revealing private details of that transaction or event.
Zerocoin, also known as Zerocoin protocol, is originally a proposal to give Bitcoin a privacy function. It is currently implemented by, among others, the coins Zcoin and PivX. Read more about this in this detailed description of zerocoin.