defintion of "stellar velocity dispersion"

I have a naive question on how exactly astronomers define stellar
velocity dispersion.
I was trying to understand some of the classic papers indicating
correllation betwene "velocity dispersions"
such as Faber-Jackson relation and M-sigma correlation
and such papers. However one thing which is not clear to me and is how
is "velocity dispersion"
defined ? I tried to read some papers and could not find any clear
discussion. Is it just the rms velocities
of all stars in the galaxy , or is it something lot more subtle.
Thanks