What is the mirror boundary command in Lammps?

Dear "Lammps-group"

I’m simulating a Nano droplet and I need a mirror boundary condition
at top of box. This condition means
that any atom reaching the top boundary continues with the same x and
y velocities, but reverses its z velocity.
So what boundary command must I use in y direction?
Thanks for your help.
Sincerely,
Mohsen Ghorbani

Sounds like you’re describing a wall. See fix wall/reflect.