mauricio estimates that the first module of the project could be completed in as few as 15 days or could take as many as 25 days but most likely will requrie 20 days. what is the 20 day estimate called?

PERT is "a statistical tool used in project management" (Program evaluation and review technique (2020), in Wikipedia), and it is commonly used with CPM (Critical Path Method) to manage projects.

Inside PERT, there are different defined times to accomplished an activity in a project, that is:

An optimistic time or minimum time required to accomplished an activity, i.e., if everything goes better than normal, the activity is accomplished before expected.
A pessimistic time, a time quite the opposite to optimistic time.
A most likely time, or a time required to accomplished an activity if everything goes as expected or normally.
An expected time, an statistical estimation.

Considering the question, we have that the time when "the first module of the project could be completed":

"[...] in as few as 15 days" is the optimistic time.
"[...] or could take as many as 25 days" is the pessimistic time.
"[...] but most likely will require 20 days" is the most likely time.

As a result, the 20-day estimate is called the most likely time in the context of the PERT/CPM techniques.