Monotony; a lack of variation — when speech, music, writing, etc., continues in the same tone without variety.
彼のプレゼンは一本調子で、聴衆の注意を引けなかった。
His presentation was monotone and failed to hold the audience's attention.
この曲は同じリズムが続き、全体に一本調子だ。
This song keeps the same rhythm and is monotonous overall.
議論が一本調子になってしまい、反対意見が出なかった。
The discussion became one-sided and no opposing opinions were raised.