系 (xì) can mean 'to tie' or 'to fasten,' but it is also widely used as a noun meaning 'system' or 'department,' as in 太阳系 (solar system). The meaning depends on context; as a verb it means to bind or connect, while as a noun suffix it denotes a systematic grouping.
地球属于太阳系的一部分。
Dìqiú shǔyú tàiyángxì de yī bùfen.
Earth is part of the solar system.
她在大学中文系学习了四年。
Tā zài dàxué Zhōngwén xì xuéxí le sì nián.
She studied in the Chinese department at university for four years.
请把安全带系好,飞机马上起飞。
Qǐng bǎ ānquándài jì hǎo, fēijī mǎshàng qǐfēi.
Please fasten your seatbelt; the plane is about to take off.