An expedition is an organized journey that is undertaken for a particular purpose, like a group of scientists launching an expedition to South America to seek out a new medicinal herb.
An expedition is no walk in the park. It's a serious journey, often with a very specific purpose undertaken by several people and lots of equipment. Sometimes, it can even be dangerous, like when the arctic adventurer was attacked by polar bears during an expedition to the Arctic. Needless to say, he hung up his explorer's hat after that expedition.