系 (xì) means "system" or "department" and often appears as a suffix in compound words. It can refer to organized structures or networks, such as 系统 (system) or academic departments in universities.
她在大学读的是中文系。
Tā zài dàxué dú de shì Zhōngwén xì.
She studied in the Chinese department at university.
这个问题涉及多个部门和系的合作。
Zhège wèntí shèjí duō gè bùmén hé xì de hézuò.
This issue involves cooperation among multiple departments and faculties.
我们系今年招了五十名新生。
Wǒmen xì jīnnián zhāo le wǔshí míng xīnshēng.
Our department enrolled fifty new students this year.