What is an uncle/ommer block?
The Ethereum blockchain is described as containing "ommer" blocks, usually called "uncle" blocks by the general public. What is an ommer/uncle block, and why are they needed?
The term "ommer" is a gender-neutral alternative to aunt / uncle. See http://nonbinary.org/wiki/Gender_neutral_language#Aunt.2FUncle
From the glossary
Uncle: a child of a parent of a parent of a block that is not the parent, or more generally a child of an ancestor that is not an ancestor. If A is an uncle of B, B is a nephew of A.
Why they are needed?
To help reward miners for when duplicate block solutions are found because of the shorter block times of Ethereum (compared to other cryptocurrency). An uncle is a smaller reward than a full block. (And if they are submitted later than the next block, the reward rapidly diminishes, ending at zero after seven blocks later.)
OMG...That sounds like a crazy definition. why can't we say uncle = sibling of the parent?