There’s no clear definition.
"Morning sickness" isn't an official diagnosis -- and it's also a complete misnomer, because it can strike women at any point during the day (and in some cases, feels a lot more like 24/7 sickness). Doctors and midwives generally prefer the term "nausea and vomiting of pregnancy," which is intentionally broad. "There's not a strict definition," Dr. Patricia Robertson, a professor in the department of obstetrics and gynecology and reproductive sciences with the University of California, San Francisco, told The Huffington Post. Which means that for some women, morning sickness will manifest itself as a mild but unrelenting queasiness; other women will throw up multiple times a day. Food aversions are also a common symptom.
"Most women," Robertson said, "will have some nausea during pregnancy."
Source: The Huffington Post, August 20, 2015