Lew Hodgett writes:
It's a neutral.

"Neutral" can be a very confusing term.

If you're confused by the term "neutral", hire an electrician.

Rather than neutral, think of it as an "above ground return".

Using the word "ground" to describe the center tap of a transformer is
misleading. It is the neutral tap of a multi-tap transformer. It is
grounded at some point, but it is not ground, nor is it a "return".