A transaction sent to the mempool that has not yet been confirmed in a block. Security of 0-conf transactions vary by blockchain.
1.1 Bitcoin Cash (BCH)
On the Bitcoin Cash (BCH) network, once a transaction enters the mempool it's not possible to re-spend those funds. To cause a 0-conf transaction on Bitcoin Cash (BCH) to be insecure the spender would need to collude with miners, which is not an easy task.
Since 0-conf transactions are safe and secure on BCH, it is an ideal blockchain to be used as cash in everyday life.
It should be noted that it is possible to double-spend a 0-conf transaction with a large amount of effort and coordination, so for any significant transfers or purchases - i.e. over $2,500 USD - we recommend waiting for at least 2-3 confirmations.
1.2 Bitcoin (BTC)
On Bitcoin (BTC) when transactions are in their 0-conf state the transaction cannot be considered complete if the RBF flag is set, or the fee is low in a large mempool. This means that until the transaction has a least 1-2 confirmations, it is possible to re-spend those funds. Double spending like this still require some effort, but a business should be wary of accepting a Bitcoin (BTC) zero-conf transaction for anything more than nominal transactions.
* All terms and definitions may update as the Cryptionary improves.