Something arbitrary is decided on a whim, with no reason that anyone can point to. An arbitrary deadline, an arbitrary line on a map, an arbitrary rule that nobody can explain.
It is nearly always a criticism, and the criticism is specific: not that the decision is wrong, but that nothing justifies it. Random is the word learners often reach for instead, and it is not the same — random means without pattern, arbitrary means without reason, and a decision can be arbitrary while being entirely predictable. Mathematics and computing use a neutral sense: an arbitrary number means any number you like, chosen for the sake of argument. Arbitrary power, in law and politics, means power with nothing to check it.
1adjectivearbitraryDecided with no reason behind it.
“The cut-off date looks completely arbitrary.”
synonyms: random, capricious
2adjectiveusingUsing power without limit or check.
“The law exists to prevent arbitrary arrest.”
3adjectivemathsIn maths or computing, any value chosen freely for the sake of argument.
“Take an arbitrary point on the line.”
arbitrary is a quality of a decision.
Also accepted in games: random
Latin arbitrarius came from arbiter, a judge — someone called in to settle a dispute. An arbitrary decision was once simply the arbiter's decision, his to make. The disapproval came later, when people started noticing what an unchecked judge could do.
Latin
arbitrārius
Middle English
arbitrarie
English, today
arbitrary
Built from
From Middle English arbitrarie, from Latin arbitrārius. From Middle English arbitrarie, Latin arbitrārius (“arbitrary, uncertain”), from arbiter (“witness, on-looker, listener, judge, overseer”).
Etymology from Wiktionary, CC BY-SA