Transaction Fee

It is the amount of cryptocurrency paid to a miner for validating a transaction and recording it into a block. Often it can be set freely by the user and it can be accepted or rejected by the miner. If the miner rejects it, the transaction goes back to the original originator. In Bitcoin, for example, it is released back within 72 hours.

The wallet usually helps to figure out the current average fees and the one to choose in order to make sure that the transaction does not get stuck in the mempool and then returned.