Why “相同意思的词” is called “同义词” instead of “同意词”？ Why is the abbreviation of “相同意思的词” “同义词” instead of “同意词”？
同义词 = 相同 定义(definition) 的词

aka words with same definition.

As for why 同意词 isn't used, I cannot find any authoritative source, however I believe it's mainly to avoid conflict with the word 同意 (v. agree, n. agreement).