trimester

American  

[trahy-mes-ter, trahy-mes-] / traɪˈmɛs tər, ˈtraɪ mɛs- /

noun

1. a term or period of three months.

2. one of the three approximately equal terms into which the year is divided by some colleges, schools, etc.

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trimester British  

/ traɪˈmɛstə /

noun

1. a period of three months

2. (in some US and Canadian universities or schools) any of the three academic sessions

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Other Word Forms

• trimestral adjective
• trimestrial adjective

Etymology

Origin of trimester

1815–25; < French trimestre < Latin trimē ( n ) stris of three months, equivalent to tri- tri- + -mē ( n ) stris ( 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."

Example Sentences

Men are now joining women in obsessing over trimester zero, a coinage referring to the months leading up to conception.

From The Wall Street Journal • Apr. 10, 2026

Indira has already had the mosquito-borne disease, chikungunya, in the first trimester during a nationwide outbreak in Cuba.

From BBC • Mar. 24, 2026

Scientists have shown that this ratio reflects the balance of estrogen and testosterone a fetus is exposed to during the first trimester of pregnancy.

From Science Daily • Feb. 10, 2026

Two, it assumes that you haven’t been infected in your second and third trimester.

From Slate • Dec. 4, 2025

Finally, during her second trimester, Blanca didn’t even bother, more out of preoccupation with the baby than out of hopelessness.

From "Bodega Dreams" by Ernesto Quinonez