This is the 525th most frequent Portuguese word.
trimestre
A period of three months; a quarter of a year.
Here, "trimestre" is used to refer to a three-month period within a year.
O primeiro trimestre do ano foi muito produtivo.
The first quarter of the year was very productive.
In this case, "trimestre" describes one of four three-month periods in an academic year.
Durante o último trimestre da graduação, fizemos muitos seminários.
During the last quarter of the undergraduate course, we had many seminars.
Here, "trimestre" indicates a regularly recurring three-month period often used in organizational or business contexts.