a written communication
missive - note
Typically, a ‘missive’ refers to a hand-written communication, though in recent times it has also been adapted to include email. It can be something long, like a formal letter, but, particularly in a professional context, the word is used to refer to something short, like a note or memo, that is created and sent quickly as it conveys important information.
I sent a missive to staff to alert them to the recent changes in our data protection policy.
Alex wrote a lengthy missive to the committee, detailing his grievances in full.
I’m only writing a missive, so it’s acceptable to use shorthand and jargon.