Emily Dickinson's poem is an example of iambic tetrameter
There is no such thing as an iambic trimester (trimester refers to three months). The prefixes "penta", "tri" or "tetra" refers to the number of feet in each line. Petra means five, tri means three and tetra means four. An iamb refers to how each foot is arranged, an unstressed syllable which is followed by a stressed syllable. Dickinson's poem is composed of iambic trimeter and tetrameter. Each combination of an unstressed syllable followed by a stressed syllable is one foot.
Here are some examples: (unstressed stressed)
This is my letter to the world, tetrameter
The simple news that Nature told, tetrameter
With tender majesty. trimeter
