The root, origin, or source of something—the fundamental starting point or cause.
問題の根源を突き止める必要がある。
We need to get to the root cause of the problem.
彼の成功の根源は努力だと私は思う。
I think the source of his success is hard work.
その噂の根源は誰か分かるか。
Do you know who the source of that rumor is?
病気の根源がまだ解明されていない。
The origin of the disease has not yet been determined.