I'm back in mathematics / fluids after a fairly long break and feeling rusty. In the context of the Euler equations I sometimes see
$$ \mathbf{\nabla}\cdot \left(\rho \mathbf{U}\mathbf{U} \right) $$
I'm not sure what the correct interpretation of this notation is. I don't think its an implicit dot product (must be a vector inside the brackets?), nor is it a cross product ($\mathbf{U}\times\mathbf{U}=\mathbf{0}$).
You'd think this would be something I could look up, but apparently I didn't have much luck formulating the right search term to turn something useful up. I know this is likely a stupid / elementary question, so I 'd appreciate someone spelling it out for me. Thanks in advance.