a period of three months; especially one of the three three-month periods into which human pregnancy is divided
"She decided to plan her maternity photoshoot for late in her second trimester."
one of three divisions of an academic year
"The university's orientation schedule is organized by trimester, so freshmen need to check which term they are starting in."
A period of three months or about three months; quarter.
"The doctor told me that my baby's development changes significantly during each trimester of pregnancy."