electronic seal
An electronic seal is the mark inside a document that proves who issued it, and that not one byte has moved since it was sealed.
In more detail
Both words are needed. A seal on its own is what the ornament drawn in wax on a certificate is called as well, and the two have nothing to do with one another: one is a picture, and a picture proves nothing at all, because anybody can paste one in.
An electronic seal is a number worked out from the whole of the file, with a private key that one person or one company holds and never gives away. Anybody can check it with the matching key, the one everybody may see, and nobody can produce it without the private one. Change a single character in the document and the check fails.
What it does on a real document
The seal covers the whole file except the small hole its own value sits in. Reading software opens the document, reads everything on both sides of that hole, works the number out again, and shows the answer in a banner across the top of the page before anybody has read a word.
It is why a sealed document can be added to without the seal breaking: what is added is written after the sealed part, and the reader shows the document as it was approved and says what came afterwards.
Where it comes from, and why
The idea is older than the format: a way of proving who wrote a message, without the two sides ever having met and without a shared secret, was published in the 1970s.
Europe has since written the rules for what such a mark has to carry to stand up in law, and named the kind a company issues in its own name an electronic seal, beside the kind a person signs with. The mark alone is half of it: the other half is the certificate saying the key really belongs to whoever claims it.
See it on a finished document
The rules behind this word, and the texts they are read from