Uncle Block (Ommer Block) refers to the discarded block in the phenomenon when two blocks are simultaneously created, resulting in one block being omitted from the blockchain.
The uncle block (ommer block) phenomenon is relevant primarily within the Ethereum mining context. This concept takes place due to the inherent nature of the Ethereum ledger, which allows for only one block to be entered at a given time. They are also referred to as stale blocks by certain users in the blockchain space.
In any public blockchain, such as Bitcoin or Ethereum, users have open access to mining. Thus, a robust mechanism is needed to ensure sound validation for input data. The method found within the Ethereum blockchain system is one where blocks are individually registered on the ledger. In the case of a double-entry at a single point in time, the block would be counted, whereas the other would be discarded. The discarded block is essentially referred to as the uncle block and is crucial in ensuring that data alteration is avoided within the blockchain.
In order to understand why Ethereum structures its blockchain system in a manner that would give rise to uncle blocks, we need to take an in-depth look into how its data is structured. Several blockchains utilize the Merkle Tree concept based on which data blocks are linked to one another through ancestral relations, which include parents, children, siblings, and even uncle blocks.
It is important to identify that although the system rejects the uncle block (ommer block), the blockchain ensures fairness towards miners by not denying uncle block miners the reward for their efforts. Therefore, according to the coin’s commitment to a proof-of-work consensus structure, even uncle blocks are due to a specific block percentage reward along with transaction fees. This mechanism ensures somewhat of a level playing field for each of the miners, without a single party benefitting at the expense of the other. However, the uncle block reward attained through mining remains less than that of a completed block.
When a cryptocurrency decides to switch from a proof-of-work to a proof-of-stake system, the reward structure for uncle blocks takes a hit as well. In that case, the uncle block miners will be awarded only transaction fees and not a percentage of the block reward.