My thesis promotor sent me this text. It talks about automorphism of free groups. On the first page, the following is written:
Let $\operatorname{Aut}(F_2) \to \operatorname{GL}_2(Z)$ be the homomorphism induced by the abelianization of $F_2$ (the free group of rank 2). It is known that the group of inner automorphisms $\operatorname{Inn}(F_2)$ is the kernel of this map and that $$\operatorname{GL}_2(Z) \cong D_4 \underset{D_2}{*} D_6...$$
(the $D_2$ is placed immediately below the star, the star being at the position where one normally places the multiplication sign, didn't know how to fix this. If anyone does, feel free to edit)
What does this strange notation mean?
This is standard notation for the amalgamated product, a pushout of sorts.
Edit: I think it's called the amalgamated product, without the adjective free (because it is not actually "free").