Trimester is from French trimestre, from Latin trimestris ('of three months'), from tri- ('three') + mensis ('month'). Important recorded forms include trimestris (Latin), tri (Latin), and mensis (Latin). Morphologically, the word preserves a built structure whose named elements carry the historical sense into English. The modern sense continues the older lexical line without requiring a separate origin. In current use it covers a period of three months, especially of a pregnancy or academic term. The wider family includes derived forms such as trimestral and related words such as semester and bimester.