Nausea is a stomach condition in which the individual feels like they need to vomit. Sometimes this is associated with airsickness, pressure changes.
Fluctuations in Gravity have been known to cause nausea in humanoids. DS9: "Treachery, Faith and the Great River"
Ezri Dax was known to suffer from space sickness after being joined with the Dax symbiont. DS9 "Image in the Sand"