/ traɪˈmestə(r); NAmE / noun
1.
( medical ) a period of three months during the time when a woman is pregnant :
the first trimester of pregnancy
2.
( NAmE ) = term :
The school year is divided into three trimesters.
—compare semester
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WORD ORIGIN
early 19th cent.: from French trimestre , from Latin trimestris , from tri- three + mensis month.