A property of Bitcoin transactions that allows them to be replicated with another transaction id before they are included in a block.
Bitcoin transactions have a transaction id (txid) formed as a hash over the data involved in the transaction. That suggests that it is a unique identifier for a transaction.
However, the txid of a transaction is only unique once the exact data in the transaction has been finalized by being incorporated into the blockchain (and confirmed). Until then, there are hacks that allow altering the underlying data and hash. This is not a security issue because it is not possible to alter how many bitcoins are transferred from what input to what output. But details such as what format the transaction’s cryptographic signature takes, or the exact signature script used, can be changed: They are malleable, making the derived hash or transaction id malleable as well.
Wiki derived from answer on What is Transaction Malleability?