What is an Epoch on Zenon Network?

Today, an "Epoch" in Zenon Network refers to a distinct period of time. In the code today, an epoch is (24 hours long). The end of an Epoch is what triggeres the release of rewards to validators in the network.

The term "Epoch" is defined in the Whitepaper.

Epoch - The transactions are grouped in consensus rounds called epochs. In every epoch, each of the nodes that participate in the consensus algorithm must compute a PoW with adjustable difficulty. The finish of a PoW is marked by a special transaction, which is then sent through the network via broadcast. After receiving the finishing PoW transaction from ζ, the node enters in the next epoch and marks this with a particular transaction.