SI units are the International System.

SI 单位是国际单位制。

To avoid any confusion, in science we use what are called SI units.

为了避免任何混淆，在科学中我们使用所谓的 SI 单位。

It is the only thing in the entire universe with a mass of exactly one kilogram because it IS the kilogram. It is also the only SI unit that is still defined by a physical object.

这是全宇宙中唯一一个质量为一公斤整的物质，因为它就是“一公斤”。 公斤同时也是唯一一个用一个指定物质定义的国际主单位。