trimester
Americannoun
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a term or period of three months.
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one of the three approximately equal terms into which the year is divided by some colleges, schools, etc.
noun
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a period of three months
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(in some US and Canadian universities or schools) any of the three academic sessions
Other Word Forms
Derived Forms
Etymology
Origin of trimester
1815–25; < French trimestre < Latin trimē ( n ) stris of three months, equivalent to tri- tri- + -mē ( n ) stris ( see semester)
Explanation
When something lasts for three months, you can say it lasts for a trimester. Each three-month period of pregnancy, for example, is one trimester. A trimester is typically three months, including a trimester of human gestation (or the time pregnancy lasts), although sometimes it's a length of time divided into three equal parts. An academic calendar is usually split into two semesters, but when it's made up of three sections, each is a trimester. The Latin root of trimester is trimestris, "of three months," from tri-, "three," and mensis, "month."
Vocabulary lists containing trimester
Far from the Tree
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tri-
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tri (three)
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
But from 2021 to 2024, prenatal care starting in the first trimester declined to 75.5 percent.
From Salon • Mar. 1, 2026
"I literally spent the first whole of my first trimester in bed on ice cubes, because I couldn't eat or drink," she said.
From BBC • Jan. 21, 2026
There was not enough consistent data to examine whether risks differed by trimester of exposure, the sex of the baby, or how frequently acetaminophen was used.
From Science Daily • Jan. 19, 2026
Late in her third trimester, doctors diagnosed her with placenta accreta, a potentially life-threatening condition in which the placenta grows too far into the wall of the uterus.
From The Wall Street Journal • Dec. 28, 2025
She’d nearly miscarried in the first trimester, and now that she was approaching twenty weeks, she was beginning to have contractions.
From "Educated" by Tara Westover
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
