In “DC Universe,” the “DC” stands for Detective Comics.

That comes from the original comic-book title Detective Comics from the 1930s, which became so popular (especially with Batman) that the publisher eventually took “DC” as its brand name, and the superhero setting built around those characters became known as the DC Universe.

So, when you see “DC Universe” or “DCU,” you can read it as the “Detective Comics Universe.”

TL;DR: “DC” = Detective Comics.