表妹 refers to a female cousin who is younger than you.
表妹 (biǎo mèi) cheat sheet for homework
外甥女 (wàishēngnǚ): This term refers to your sister's daughter, your niece.
侄女 (zhínǚ): This term refers to your brother's daughter, also your niece.
堂妹 (táng mèi): This term refers to a female cousin who is the daughter of one of your parents' siblings.