break·through
ˈbrāk-ˌthrü
plural breakthroughs
1
a
: a sudden advance especially in knowledge or technique
a medical breakthrough
b
: a person's first notable success
—often used before another noun
a breakthrough novel
the actress's breakthrough performance
2
a
: an act or instance of moving through or beyond an obstacle
a breakthrough in the talks between the region's leaders
a breakthrough agreement
b
: an instance of passing through a barrier or protection
The ice under the surface begins to deteriorate, increasing the risk of breakthroughs.—Vicki Winger
specifically, medical
: infection occurring in someone who is fully vaccinated against an infectious agent
A vaccine breakthrough occurs when a person tests positive for COVID-19 two weeks after being fully vaccinated against the disease … —David Rasbach
—often used before another noun Two other CDC studies of breakthrough cases at nursing homes showed the vaccines protected residents, even when the virus spread through the homes. —Maura Dolan
… the academy recently recommended that children receive two doses of the chicken pox vaccine, rather than just one, because the additional dose bolsters immunity and may help reduce breakthrough infection. —Maria Raven
3
warfare
: an offensive military assault that penetrates and carries beyond a defensive line
broke through; broken through; breaking through; breaks through
: to make a breakthrough
Cite this Entry
“Breakthrough.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/breakthrough. Accessed 15 Jul. 2026.
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