Security Mindset
Threat Model
Also: threat modeling
A short written answer to: what are we protecting, who might attack it and why, how, and what will we do about it.
In more detail
A threat model replaces the unanswerable question 'is this secure?' with a specific one about particular assets, actors and paths. It can start as a page of notes and a rough diagram, and it is most useful early, before a system is opened to others.