1. OG = Original = 1st Pressing
2. Reissue, Repress, Rerelease = The same in my opinion. Same meaning, term and outcome. It doesn't matter if it's on the same label, who manufactured it or anything of the sort. What matters is the fact that they were pressed with the consent of the Executive Producers acknowledgment/consent. ***Most artist don't have the rights to their own projects, because they don't own the rights to the project. If the artists is in fact the Executive Producer and the artist at the same time, then the artist can grant the rights to re-manufacture and distribute the album, because he is the Executive Producer.
3. Bootleg = A pressing done without written consent, contract and/or permission of the Executive Producer. This also means that an artist can be a bootlegger too, if he doesn't own the rights to the album. Yes, he's the artist, but if he/she/the artist isn't the actual Executive Producer, then he/she/the artist doesn't own the rights to the album and its publishing, which means that he can't give permission to manufacture anything. The artist can give you his consent, but his consent means nothing if he's not the actual Executive Producer of that album. Basically if the Executive Producer didn't grant the manufacturing and reproduction of the material, It's a bootleg. This means that you can get sued, even if you got the artists consent. The artist better be the owner/Executive Producer/owner of his publishing, because you will get sued and you are a bootlegger. If the artist is the Co-Executive Producer, you will still need a contract/written consent by the other Co-Executive producer. Without consent from all Executive parties, your project is in fact a bootleg. The key words are: Executive Producer & Contract/Written Consent. If you are not that, have not received this from this person in this position and don't have a contract/written consent, then you are in fact a bootlegger.
4. Pressing Number = The order in which they were released. 1st pressing always = OG, but everything else falls in the order of manufactured date. An example of this is an album that was re-release/re-pressed/re-issued twice in one year. If the 1st repress came in the month of February, it's the 2nd Pressing, because it's not the OG/Non Reproduction of any form. If the second repress came in November of the same year, it means that it will be the 3rd Pressing. The number of pressing is determined by the order of the date's in which that repressing occurred.
That's as simple as I can put it.