Tour, excursion, trip, journey. Which of these words is the odd-one-out? Here's a clue: it's the one which means traveling ONE WAY, especially returning home after a couple of years' stay in a foreign country, whereas the other three all mean traveling BOTH WAYS, outward and return without a long break, and doing something along the way, like visiting a place, seeing the sights, or doing some business.

The odd-one-out is "journey", from the French word for the distance which could be traveled (by horse, say) in one day. It's wrong to think that a trip is only for fun; a trip means "go somewhere, do something there, and come back again" - so a sight-seeing trip (i.e. an excursion) means to choose a destination, go there, look at the sights and come home again, and a business trip means to travel to another town to visit a company or a trade fair, do some business, and travel home again. A tour is a kind of trip which goes round a defined route, stopping to look at interesting places on the way, and returning home again at the end.