tri·mes·ter 
/traɪˈmɛstɚ/ noun plural tri·mes·ters
[
count]
1 : a period of three months ;
especially : one of three periods into which a woman's pregnancy is often divided
▪ development in the first/second/third trimester
2 US : one of three periods into which an academic year is sometimes divided —
compare quarter, semester, term
