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
Skin also dries out more easily, lips crack and some people notice more frequent nosebleeds or additional sinus discomfort.
—
Daryl Austin, USA Today, 13 Dec. 2025
Other reasons to contact a health care clinician include trouble catching your breath, persistent wheezing, severe headaches, one-sided ear pain, sinus pain lasting more than a few days or dehydration.
—Katia Hetter, CNN Money, 4 Dec. 2025
In Bell’s case, it was caused by her initial cold, which then devolved into a sinus infection, which then became orbital cellulitis that had begun spreading into her brain.
—Cara Lynn Shultz, PEOPLE, 3 Dec. 2025
However, most colds and sinus infections are caused by viruses.
—Health, 3 Dec. 2025
See All Example Sentences for sinus
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Daryl Austin, USA Today, 13 Dec. 2025
Other reasons to contact a health care clinician include trouble catching your breath, persistent wheezing, severe headaches, one-sided ear pain, sinus pain lasting more than a few days or dehydration.
—Katia Hetter, CNN Money, 4 Dec. 2025
In Bell’s case, it was caused by her initial cold, which then devolved into a sinus infection, which then became orbital cellulitis that had begun spreading into her brain.
—Cara Lynn Shultz, PEOPLE, 3 Dec. 2025
However, most colds and sinus infections are caused by viruses.
—Health, 3 Dec. 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 17 Dec. 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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