Nausea was a stomach condition in which the individual felt like they needed to vomit. Sometimes this was associated with motion sickness, pressure changes, or excessive g-forces.
In 2153, Dr. Fer'at, a psychiatric analyst posing as a pathologist, examined Captain Jonathan Archer for signs of pyritic radiation. Fer'at asked Archer if he had experienced symptoms of nausea or dizziness. (ENT: "The Expanse")
Dr. Phlox prepared anti-nausea medication for Charles Tucker III and Hoshi Sato after Sato vomited in the decontamination chamber's toilet on Enterprise NX-01. (ENT: "Observer Effect")
Lieutenant Malcolm Reed experienced nausea during the Shuttlepod 1 mission to Mars in 2155. Doctor Phlox gave him an organic waste bag which he used. (ENT: "Terra Prime")
When asked by Hikaru Sulu what he thought of how they were changing course so much, Ensign Pavel Chekov replied, "I think I'm getting space sick." (TOS: "Amok Time")
After Spock was shot by a poison flower and being injected with medicine, Kirk asked, "Spock, are you all right?" to which Spock replied, "Dr. McCoy's potion is acting like all his potions, turning my stomach. Other then that, I am quite well." Then McCoy said to Spock, "If your blood were red instead of green, you wouldn't have an upset stomach." (TOS: "The Apple")
During The Doctor's delusions on USS Voyager's holodeck, he asked a holographic recreation of Captain Kathryn Janeway if she were experiencing any pain, dizziness or nausea following a supposed Kazon attack, to which she replied that she had a headache and felt disoriented. (VOY: "Projections")
Worf had a very weak stomach when it came to zero gravity, becoming ill while attempting zero-gravity combat training at Starfleet Academy. (Star Trek: First Contact)
Fluctuations in gravity were known to cause nausea in humanoids. (DS9: "Treachery, Faith and the Great River")
Ezri Dax was known to suffer from space sickness, a condition which could cause nausea, after being joined with the Dax symbiont. (DS9: "Image in the Sand")
B'Elanna Torres felt nauseous after Voyager was infected by a macrovirus in 2373. Tom Paris was surprised by this since Klingons possessed a redundant stomach. (VOY: "Macrocosm")
In 2375, The Doctor told Neelix that nausea was one of the symptoms of nihiliphobia, a condition Neelix was suffering from while Voyager was trapped in the void. (VOY: "Night")