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

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

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