My definitions:
Original - first legal press. Applies to local and later national releases, both original. Releases in other countries can be originals too if issued around the same time, e.g. within 2/3 years after the initial issue, e.g. UK Stateside issues
Reissue - much later legal re-release due to demand, be it from the Northern scene or Doo Wop scene or whatever. Independent of country of origin, e.g. most UK Grapevine releases are reissues even though they're first UK releases.
Pressings/Bootlegs: both the same, illegal re-releases. Or illegal presses of previously unissued recordings, e.g. Bob Dylan's infamous basement recordings.
Counterfeits: Pressings/Bootlegs made with every effort to look like the original presses.
Have I forgotten anything?
Edit: have amended the "pressings/bootlegs" bit.