A trimester is a period of three months, especially when dividing something into three equal parts.

You'll most often hear this word in two contexts. In schools, some districts divide the academic year into three trimesters instead of two semesters or four quarters. Each trimester lasts about three months, so you might have final exams and report cards three times during the school year rather than twice.

The word is also commonly used during pregnancy. A baby develops over roughly nine months, and doctors divide this time into three trimesters. The first trimester covers months one through three, the second trimester covers months four through six, and the third trimester covers months seven through nine. Each trimester brings different changes and developments as the baby grows.

Think of a trimester as cutting something into thirds based on time. Just as a semester is one part of a school year, a trimester is one of three parts. If your school uses trimesters, being in the second trimester means you are in the middle third of the year.