Since its publication in 1938, Nausea has become a central term for describing the pain associated with existential philosophy. More specifically, the term describes the sickness that Antoine Roquentin feels when he realizes that beneath all of the pretty words he uses to describe the world, there's just brute, naked existence that doesn't care about him or anything he does.
As iconic as the title Nausea went on to become, though, it wasn't actually Sartre's first pick. He originally titled the book Melancholia, but his publisher felt that it didn't enough oomph to get people's attention. After all, we don't want readers thinking that they're about to read 170- pages of whining despair; do we? Oh, wait…
After his manuscript was rejected a first time, Sartre decided to rename his book, The Extraordinary Adventures of Antoine Roquentin. In this case, he must have been joking, since Nausea doesn't contain anything remotely close to an adventure. The title was no doubt ironic, since Nausea is often boring and meandering on purpose, in order to make its point about the pain of being alive.
Eventually, Sartre agreed to the title Nausea. It wasn't his choice, but his publisher liked it. Sartre didn't like the word because he felt that people would associate Antoine's illness with something physical, like real nausea, rather than the philosophical pain that Sartre was trying to explore.
In the end, though, it seems like the publisher made a good call. Instead of having people misread the term nausea, Sartre succeeded in redefining this word for the twentieth century. From 1938 onward, readers would continue to associate it with the "pain of existence" that Sartre explored through the story of Antoine Roquentin.