Maxim
Etymology
From Medieval Latin "maxima," short for "maxima propositio," "the greatest or foremost premise," from Latin "maximus," "greatest." A maxim was the highest, most general principle from which lesser ones followed. English narrowed it toward practical conduct, and Kant later made "maxim" the technical name for the personal principle behind an action — the rule you would, in effect, be willing to live by.
noun · /MAK-sim/
A concise rule or principle of conduct, expressed as a general guide to behavior.
What it names
The personal rule you carry into decisions — the compressed policy that saves you from re-deriving your ethics every time a situation repeats.
Example
Her governing maxim was to never make a permanent decision in a temporary mood.
Related idea
A maxim prescribes conduct; a truism only describes a state of affairs, and one so obvious that it guides nothing.