出 (chū, "to go out, to exit") + 发 (fā, "to send off, to depart"). The compound first appears in classical Chinese texts meaning "to send troops out," and gradually shifted in modern Mandarin to the everyday sense of beginning any trip or journey.

Travel is a huge theme in Chinese culture — the Spring Festival migration (春运 chūnyùn) is the largest annual human movement on Earth. 出发 is the natural word you'll hear at train stations, airports, and tour-group send-offs. Don't confuse it with 离开 (líkāi, "to leave/depart a person or place"), which emphasizes separation rather than the start of a journey.