What is the first in a trilogy called?

There's no official or universally accepted name for the first book in a trilogy. However, the most common and intuitive way to refer to it is simply as:

* The first book in the trilogy

* Book 1 of the trilogy

You could also use the name of the specific trilogy, like:

* The first book in The Lord of the Rings

* Book 1 of The Hunger Games trilogy

There are no established terms like "prologue" or "prelude" for the first installment of a trilogy, as those terms typically refer to different types of literary structures.