grapefruit is “西柚” because for most Chinese people, this type of fruit is originally from foreign countries. 西means from Western Country. In China, 柚子 is a similar fruit as 西柚but they are different. 柚子is bigger, the size is like a football. The peel of 柚子is thicker.

Normally we say grapefruit (西柚）， and pomelo (柚子）

We call it is 柚子, "西柚" and "柚子" two fruit is not the same fruit. "西柚" is another kind of fruit the Chinese people also call it "葡萄柚"。

Hi Vika, I agree with Kelly. 柚子 is more like pomelo. It's a common fruit in China (Here's a image: http://imgs.ntdtv.com/pic/2015/10-2/p6940391a932062773.jpg) Grapefruit is more like 西柚. It's not seen very often in Chinese supermarket.

柚子 is more like pomelo. Grape fruit in Taiwan, we call it 葡萄柚。 (Grape=葡萄)