noun
the same opinion; agreeing view
Used to express that one holds the same opinion as someone else. Often appears in phrases like 同意見だ or 同意見です.
その点については私も同意見です。
I am of the same opinion on that point.
彼と同意見だったので、すぐに話がまとまった。
Since I shared his opinion, the discussion was quickly settled.
賛成 means 'approval' or 'agreement' with a proposal or idea, often used in voting or decision-making contexts. 同意見 simply states that one's opinion is the same, without necessarily implying active support.
Compound of 同 (どう, 'same') + 意見 (いけん, 'opinion'). A straightforward Sino-Japanese word.