A queasy, sick feeling in the stomach like one might vomit; nausea (a symptom that may or may not lead to vomiting).
船に乗るといつも吐き気がする。
I always feel nauseous when I ride a boat.
妊娠初期は吐き気が続くことが多いと言われている。
It's said that nausea often continues during early pregnancy.
昨夜の薬の副作用で吐き気がする。
I feel nauseous from last night's medication side effects.