A person, divinity, or authoritative text which decides.
A Turing machine that halts regardless of its input.
An event or action which decides the outcome of a contested matter.
A person who rules or governs; someone or something that exercises dominion or controlling power over others.
A (usually rigid), flat, rectangular measuring or drawing device with graduations in units of measurement; a straightedge with markings.
To beat with a ruler (as a traditional school punishment).