Question

What is a diasteriomer?

Answer 1

Diastereomers (sometimes called diastereoisomers) are a type of a stereoisomer.
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Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other.
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When two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two.
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Answer 2

It's a type of stereosoimer
Or
It's the isomers which are nor superimposable and not related to each other as mirror image are called diastereomer.
It occurs when two or three more stereosoimer of a compound have different configuration at one or more of the equivalent stereocenters.