The everyday arbitrary meaning is "decided by chance or personal choice, not by a reason or a rule". If your manager picks a deadline of Thursday for no clear reason, that deadline is arbitrary; it could just as easily have been Wednesday or Friday.
The word also has a sharper sense for power used without limits, as in an arbitrary arrest, and a neutral technical sense in math and science. This guide sets out each sense, the definition of arbitrary used in law, the Latin root and the words that mean the opposite.
Entry at a glance
Meaning
Chosen by whim or chance; not based on reason, a system or fair rulesPart of speech
AdjectivePronunciation
AR-bi-trer-ee (US); AH-bi-truh-ree (UK)Word forms
arbitrarily (adverb), arbitrariness (noun)Origin
Latin arbitrarius, from arbiter (judge); in English from around 1400Opposite
reasoned, principled, rational, consistent, rule basedRegister
Neutral to formal; common in law, school and work
Define arbitrary: three senses
- Random or unreasoned. Chosen without a real reason. The cut off of 60 marks seemed arbitrary to the students.
- Unfair use of power. Done by someone with authority who follows no rules and answers to no one. arbitrary rule, arbitrary detention, arbitrary fines. This sense is clearly negative.
- Any one at all (math and logic). Not a fixed choice, so the result holds for every case. Take an arbitrary point on the line. This sense is neutral and simply means "pick any".
What links them is the absence of a governing reason. In sense 1 nobody had a reason; in sense 2 the person in power did not need one; in sense 3 the reason does not matter, because the claim works for every option.
Arbitrary meaning in sentences
- Our landlord's rules about guests feel completely arbitrary; they change every week.
- The referee's arbitrary decisions angered both teams.
- We drew an arbitrary line on the map just to split the work into two zones.
- The court ruled that the officer's search was arbitrary and therefore unlawful.
- For an arbitrary whole number n, the sum of the first n odd numbers equals n squared.
A teacher says phones are banned on Tuesdays but allowed on Wednesdays, and cannot explain why. Students call the rule arbitrary. The word fits because nothing about Tuesdays makes phones more harmful; the choice seems to rest on a whim.
Arbitrary in law and rights
In legal writing, arbitrary is a serious charge. It describes decisions that ignore facts, reasons or fair procedure.
- Human rights. International human rights texts state that no one should face arbitrary arrest, detention or exile, meaning arrest without lawful grounds and fair process.
- India. The Supreme Court of India has long held that arbitrary action by the state can violate the right to equality in Article 14 of the Constitution.
- United States. Courts reviewing government agency decisions can strike them down as "arbitrary and capricious" if the agency failed to give a reasoned explanation.
From judge to whim: the word's history
Arbitrary comes from Latin arbitrarius, connected to arbiter, a judge or someone chosen to settle a dispute. English used it from around 1400 for matters decided by a person's own judgment rather than fixed law. Since that judgment could be wise or careless, the word slowly slid toward "based on mere opinion" and "uncontrolled by law", both recorded by the 1640s. The same root gives arbitration and arbitrator, which keep the old, positive idea of a fair judge.
Arbitrary vs random vs subjective
| Word | Main idea | Example |
|---|---|---|
| Arbitrary | No good reason behind a choice | an arbitrary deadline |
| Random | Chance, with no pattern or plan | a random draw |
| Subjective | Based on personal feeling or taste | a subjective review |
| Capricious | Changing suddenly on impulse | a capricious boss |
A random choice can be fair, like a lottery for seats. An arbitrary choice suggests someone could have used a reason and did not. Opposites include objective, reasoned and consistent. Being arbitrary is close to showing bias when the whim always favors the same side, and the reverse of careful discretion.
Tips for using arbitrary
- Use it to criticize a rule or decision that lacks a reason: an arbitrary policy.
- Put it with numbers and limits: an arbitrary figure, an arbitrary cut off.
- The adverb is arbitrarily: prices were arbitrarily raised.
- Do not confuse it with ambiguous, which means unclear. A rule can be crystal clear and still arbitrary.
- In US speech the word has four syllables, AR-bi-trer-ee; in UK speech it is often squeezed to AH-bi-tree.
For more useful adjectives, browse our everyday vocabulary words, or compare with ambiguous and vague.
Frequently asked questions
What does arbitrary mean in simple words?
Arbitrary means chosen without a good reason, as if on a whim. An arbitrary rule is one nobody can justify.
Is arbitrary a negative word?
Usually, yes, when it describes rules, decisions or power, because it suggests unfairness. In math it is neutral and just means any value you choose.
What is the opposite of arbitrary?
Reasoned, principled, rational, objective or consistent. Each suggests a choice that follows clear grounds or rules.
What is the difference between arbitrary and random?
Random means by chance, with no pattern. Arbitrary means no good reason was used, often by a person who could have used one. A fair lottery is random but not arbitrary in the unfair sense.
What is an arbitrary number?
It is a number picked without any special reason, such as setting a limit of 50 just because it sounds round. In math, it can also mean any number at all, so the rule works for every case.
What is arbitrary arrest?
It is an arrest made without lawful grounds or proper process. Human rights law treats freedom from arbitrary arrest as a basic right.