Some people also talk about a
galactic habitable zone.
This is the region in the galaxy where
the conditions might be appropriate
for life to originate. This is obviously
a much less rigorous definition.
This kind of like the Goldilocks problem. Too close to
the galactic center and the galaxy is too crowded --
this means lots of supernovas. Too far out in the galaxy
and there is not (or has not been) enough star formation --
not enough heavy elements. In the middle, near (but not
too near) star forming regions (which is what happens
in the spiral arms) might be "just right."
An outstanding question is therefore how big
is the galactic habitable zone and how many
planets are there in it?