the quality of being level and without curved, high, or hollow parts

The flatness of the desert was broken only by a few large piles of rocks. 沙漠看上去很是平坦，仅有几大堆石头点缀其中。

the fact that someone or something shows no emotion or interest

All the critics remarked on the flatness of the performance. 所有的评论家都谈到这场表演平淡乏味。