trimester

A trimester is a period of about three months. Most people know the word from pregnancy, which is often divided into the first, second, and third trimester. It can also describe one of the three parts of a school year in a trimester system, instead of two semesters. You might say your sister is in her second trimester or that a college uses trimesters. In both cases, the idea is the same: time is divided into parts that are about three months long.

The word "trimester" most commonly refers to one of the three periods of about three months that make up a human pregnancy. The first trimester often includes early physical changes and symptoms such as tiredness or morning sickness. The second trimester is often seen as a more comfortable stage, and the third is the final stretch before birth, when the baby continues to grow and the body prepares for labor. Doctors and expectant parents often use this term when talking about development, symptoms, and important stages of pregnancy: "She's in her first trimester," or "Many people feel the baby move during the second trimester."

However, the term is not limited to pregnancy. It can also mean one of the three parts of an academic year. Some schools and universities use trimesters instead of semesters, so the year is split into three terms rather than two. You might hear: "The university operates on a trimester system."

The word comes from Latin and is built from parts that relate to "three" and "months." That history matches its meaning well: it describes a block of time that lasts about three months. In everyday English, the word is mainly used for pregnancy and, less often, for school calendars. Whether someone is talking about a stage of pregnancy or a term in the school year, "trimester" points to a time period of roughly three months.