A departure is the act of leaving somewhere. Think about an airport that has departure gates for outgoing passengers and an arrival area for people flying in.
Since departing means to leave, a departure is an act of leaving. There are many type of departures, such as when you go to the store or take a flight. A departure can be a metaphor too, as in a departure from the norm, like if a TV cop show suddenly turns into a vampire show. If a comedian starts being serious, that's a departure too. This word is also a euphemism for death, as in "a departure from life."