I am confused in a lot of applications for academic jobs, such as assistant professor, lecturer, instructor, at academic institutions. Many of them ask for a "copy of transcript," but I am unsure about what this means. Does this mean I need to get my official sealed transcript, open it, and copy it, or does it mean I can use the unofficial transcript?
To know for sure, the easiest thing is to email the application contact person for a given job and ask.
My impression is that in many cases, the committee reviewing the transcripts does not really care whether they are official or unofficial, photocopied or not, whatever. All contain the same information. If you reach a later stage of the application process, they may require an official transcript to satisfy bureaucratic requirements.
As W88's answer says, every printed version of your transcript is "a copy of your transcript". "A copy of your transcript" doesn't necessarily mean a photocopy, it just means "a piece of paper with your transcript printed on it". If you receive a sealed transcript from your school, that is "a copy of your transcript" and you can send it in as-is.
May I ask what you mean when you say official and unofficial?
If it helps, the copies issued from the institution where you graduated are all considered official. The original document you receive is also a copy called the original copy. You can also have multiple original copies.
So when they ask for an official copy of your transcript, as long as it has the signature of your registrar or equivalent, it is official; whether it is the original or the photocopy is a different matter.