What's referred to as a "Unibody" mac?

The tag here is pretty authoritative to explain how unibody came about on the MacBook line where a united piece of aluminum was machined to house all the internal parts. This differs from the previous construction where discrete parts were assembled / welded / screwed and placed together.

This process is also used on non portable computers, but the difference in stiffness and reduction of size was most dramatic on the MacBook and MacBook Pro lines which then enabled the entire iPhone / iPad / Watch lines to evolve and refine what the unibody manufacturing changed on portables.

Apple doesn’t flat out define the term in the February 2019 Apple Style Guide, but the transition was when the original black keyboards arrived on the scene for the intel line MacBook Pro (initially) and later to the Air and MacBook lines. Currently all MacBook are considered unibody.