No, not the game engine :)
Unity builds are a method to speed up compilation time by avoiding
redundant compilation of headers. The idea is to
.cpp, etc.) files into a single source file
so that they form one translation unit instead of multiple ones. This
means that headers used by multiple of these initial source files only
have to be compiled once, which can speed up compilation time
significantly. Especially notable improvements can be made with template
heavy C++ code, that would otherwise increase the size of the separate
translation units with redundant template instantiations.
In Blender, unity builds are created with the help of CMake's unity build support.
Enabling/Disabling Unity Builds¶
Unity builds can be toggled via Blender's CMake option
Implementing Unity Builds for a Module¶
Preventing Naming Collisions¶
By merging translation units into one, there is an increased chance of naming collisions, when multiple source files define the function name/signature or type name more than once. Therefore the style guidelines include a preferred way to avoid that.