因为这些星系的质量分布在更大的区域，它们每单位体积的恒星数量比传统星系要少得多。

Because these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies.

youdao

它指出了，单位区域内源的总量。

It tells you the amount of sources per unit portion of the region.

youdao

看这儿，左边的这个是单位时间内,从区域里出来的流量。

Physically, see, this guy on the left is the total amount of stuff that goes out of the region per unit time.

youdao

So the repeating unit in DNA is this structure here, a nucleotide, which has three different regions.

NA的重复单位就是,核苷酸,它有三个不同的区域

耶鲁公开课 - 生物医学工程探索课程节选