graduate student和post graduate student到底哪个是研究生？好像两个都有用？

graduate student=post graduate student
这两种称法指代相同，只是用于不同的地区。

维基百科的注释（大意）：
研究生教育Postgraduate education（在北美通常称为graduate education，有时候称为第四级教育）是获取学位以及其他资格的一种教育，它要求在已经获得一个学位或学士学位的情况下攻读，通常被视为高等教育。
【原文】
Postgraduate education (often known in North America as graduate education, and sometimes described as quaternary education) involves studying for degrees or other qualifications for which a first or Bachelor's degree is required, and is normally considered to be part of tertiary or higher education. In North America this level is generally referred to as graduate school.

附：另有undergraduate(本科生)

post graduate student 是大专生

graduate student这个是研究生