means A period of three months, especially one of the three divisions of a pregnancy or an academic year.

from From Latin trimestris, 'of three months,' built from tri- 'three' and mensis 'month' — the same mensis that lurks in 'menstrual' and is a cousin of the Moon-words across Indo-European languages, since early calendars counted time by lunar cycles. It entered English by way of French trimestre, first describing spans of the year before doctors borrowed it to carve up pregnancy.