The cost of cryptocurrency mining at a particular moment in time. Mining difficulty will fluctuate depending upon what is going on with the blockchain (network). This will affect the transaction fees users have to pay to send coins or tokens from their wallets.
We can see this determined by the following:
Number of transactions that are trying to be confirmed at any single moment in time, divided by the total power of the nodes on the network at that time, defines the difficulty.
The higher the difficulty, the greater the transaction fee – this is a fluid measurement that moves over time.
This is why the fees on the Bitcoin and Ethereum networks can be volatile.