1. A trip or journey that is organized to accomplish a specific objective: The biologists are on an expedition into the African jungle to study certain animal species.
2. People who participate in organized trips with defined goals: The expedition had to sleep in tents while they were in the desert during their explorations.
3. Etymology: from the Latin verb expedire, which originally had the meaning: "free one’s feet" from a snare, for example.

An expedition is often a group of people sent on some important undertaking and so their feet must be free to move without hindrance.