mere; simple; simply; indicating that something is only what is stated and nothing more significant.
それは単なる噂に過ぎないから気にしないでいい。
That's merely a rumor, so you don't need to worry about it.
単なる偶然だっただけだと思う。
I think it was simply a coincidence.
単なるミスで済ますわけにはいかない問題だ。
This is not a problem that can be brushed off as a mere mistake.