What is the "Definition of what is being built"

You might be referring to the Increment, however, I've never heard the term 'what is being built'. I'd think that question could be answered at a high level with the Product Strategy / Roadmap. 

The DOD would refer to the quality standards that must be met for the Product Backlog Items to be considered done or release quality. The increment (or what is built) should meet the Definition of Done. 

"When a Sprint’s horizon is too long the definition of what is being built may change"what is the definition of what is being built? is it the DOD or the increment itself?

From the context, perhaps the “definition of what is being built” refers to something which shouldn’t change during a Sprint.

Pretty much anything can be inspected and adapted during a Sprint time-box, but one thing is immutable. Do you know what it is?

Yes the Sprint goal shouldn't be changed during a Sprint time-box

That quote is from the section of the Scrum Guide where the Sprint is described.  To me if you take the entire sentence to complete the context it makes more sense.

Sprints are limited to one calendar month. When a Sprint’s horizon is too long the definition of what is being built may change, complexity may rise, and risk may increase.

I don't try to narrow it down to a specific "thing" as much as seeing it as an explanation for why you limit the length.  You can take that sentence and apply it to anything that you build.  In the fast paced life we live, if you take too long to complete any work effort there is a risk that the problem you are trying to solve is no longer a problem.  

IS not "what is being built"   , the increment for that sprint goal ?