Is it correct to say: "(Something) is defined by...."?
Please, help me with this construction... When I need to describe a definition of something, can I use this structure? "______ is defined by____"

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Yes, you can use “______ is defined by...” because you are basically giving the word and explaining its meaning both within one sentence.

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No, you can’ t. « By » is related with a person who does this action. 
You can say « is described as_____ «

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Both "defined by" and "defined as" are possible, and they have different meanings.  A shoe might be defined as an item worn on a foot. A relationship might be defined by the mutual respect and appreciation of both parties.    If you want a little more, have a look at

https://dictionary.cambridge.org/dictionary/english/define

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