What makes a transcript official vs. unofficial? What really determines whether a transcript is official or unofficial? (I am wondering mainly in the context of US high schools and colleges.)


Is it entirely about the process by which it is delivered, i.e.
directly from an institution to a the receiver such that there can be
no tampering?   
If an issuing institution (such as a high school) does not print the
transcript on special security paper, but sends directly to the
receiver in the mail on regular paper, would this still be considered
official? 
Are there cases where an official institution would consider a
transcript to be official but an receiving institution would not consider it to be official?
An official transcript is usually sent under some sort of seal, with measures to ensure that the document has not been altered. When delivered to a student, it will usually be in a sealed envelope in such a manner as it will show proof of tampering or opening.

The usual method, though, is for the issuing school to send it directly to the intended recipient. The transcript is then authorized as "official" through some combination of stamping or seal. (Nowadays much of this can be and is handled electronically.)

There may be some schools that have specific rules that would prevent them from accepting what another school sends, but you'd have to look at individual schools' policies to know for sure.