Arbitrary
Etymology
From Latin "arbitrārius," "depending on the will of an arbiter," from "arbiter" ("judge, one who decides"). The original idea was of a matter left to a judge’s discretion. Because discretion can be exercised without stated reasons, the word slid toward "based on whim" and, in politics, "answering to no rule but one’s own will."
adjective · /AR-bih-trair-ee/
Decided by personal whim or chance rather than by reason, rule, or system.
What it names
A choice with no principled basis — an arbitrary cutoff, an arbitrary fee — where the point is that it could just as easily have been set otherwise.
Example
The passing grade was set at 63, an arbitrary line that saved some students and sank others by a single point.
Related idea
Compare nominal: a nominal fee is small and token in amount, while an arbitrary fee may be large — its flaw is that nothing principled fixed the figure.