a
: a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus
b(1)
: a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air
(2)
: a channel for venous blood
(3)
: a dilatation in a bodily canal or vessel
c
: a cleft or indentation between adjoining lobes (as of a leaf or corolla)
Examples of sinus in a Sentence
Recent Examples on the Web
When the air turns crisp in fall and winter, the dryness indoors can sneak up on you — and so can flaky skin, a scratchy throat, and stuffy sinuses (not to mention allergens).
—
Jeaneen Russell, PEOPLE, 31 Oct. 2025
Others are easily Googleable, but harmless questions asking when graduation is, if there are printing services on campus and if university health services will treat a sinus infection.
—Rachel Hale, USA Today, 29 Oct. 2025
The best down alternative pillows, however, are popular for resembling down without the dust and mites that can often wreak havoc on sinuses overnight.
—Julia Harrison, Architectural Digest, 24 Oct. 2025
Some had a fungal infection called mucormycosis, also known as black fungus, which can infect the nasal passages, sinuses, lungs and skin of people who have chronic high blood glucose or are immunocompromised.
—Carrie Arnold, Scientific American, 14 Oct. 2025
See All Example Sentences for sinus
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Jeaneen Russell, PEOPLE, 31 Oct. 2025
Others are easily Googleable, but harmless questions asking when graduation is, if there are printing services on campus and if university health services will treat a sinus infection.
—Rachel Hale, USA Today, 29 Oct. 2025
The best down alternative pillows, however, are popular for resembling down without the dust and mites that can often wreak havoc on sinuses overnight.
—Julia Harrison, Architectural Digest, 24 Oct. 2025
Some had a fungal infection called mucormycosis, also known as black fungus, which can infect the nasal passages, sinuses, lungs and skin of people who have chronic high blood glucose or are immunocompromised.
—Carrie Arnold, Scientific American, 14 Oct. 2025
See All Example Sentences for sinus
Cite this Entry
“Sinus.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/sinus. Accessed 11 Nov. 2025.
Copy CitationKids Definition
Medical Definition
sinus
noun
si·nus
ˈsī-nəs 
: a cavity or hollow in the body: as
a
: a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus
a tuberculous sinus
b(1)
: a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air
(2)
: a channel for venous blood
(3)
: a dilatation in a bodily canal or vessel
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