Arbitrary

Ar·bi·tra·ry

Depending on will or discretion; not governed by any fixed rules; as, an arbitrary decision; an arbitrary punishment.
It was wholly arbitrary in them to do so.
— Jer. Taylor.

Rank pretends to fix the value of every one, and is the most arbitrary of all things.
— Landor.
Exercised according to one's own will or caprice, and therefore conveying a notion of a tendency to abuse the possession of power.
Arbitrary power is most easily established on the ruins of liberty abused licentiousness.
— Washington.
Despotic; absolute in power; bound by no law; harsh and unforbearing; tyrannical; as, an arbitrary prince or government.

Arbitrary constant: a quantity of function that is introduced into the solution of a problem, and to which any value or form may at will be given, so that the solution may be made to meet special requirements.
Arbitrary quantity: one to which any value can be assigned at pleasure.