Here is some code I put together to search a term within a term set. This is for scenarios when you have a large term set and need to efficiently find a term with a specific label.
Here is a link which describes the exact combinations needed to make this work (Thanks Jose!)
In this code, I am searching for all terms which start with an "A" in a term set which contains locations from around the world:
Note: I am using JSOM here but this also works with CSOM