What is an Orphan Block in Ethereum?

Expect extreme competition in Proof-of-Work networks like Ethereum. 

If Ethereum developers had allowed Bitmain and other chipset manufacturers to operate at their own volition, this smart contracting platform would have been an ASIC playfield.

It didn't happen.

Ethereum is ASIC-resistant and is one of the most decentralized public ledgers.

Because block generation time in Ethereum is fast, miners can verify blocks concurrently. 

However, due to lagging, these valid blocks aren't included in the longest chain and, therefore, not part of the canonical truth.

If the network doesn't acknowledge it as valid—included as the first—or an uncle, it is orphaned—though valid in the eyes of other miners.

The only difference is that the orphan block miners (unlike uncle or the miner of the accepted block) don't receive mining rewards for their efforts. 

It happens when the block isn't confirmed as valid, seven blocks after the first block was verified and added to the longest chain. 

Since it is delayed, it doesn't become an uncle block—though it qualifies—only if it would have been validated within the seven blocks after the first block was confirmed.

There are no uncle blocks in Bitcoin. 

Instead, all of its unaccepted blocks are orphaned—miners don't receive block rewards and transaction fees for their efforts.