In legal practice, arbitrary describes a decision or action taken without a rational basis, relevant reasons or fair process-essentially, one resting on whim or caprice rather than lawful, evidence‑based judgment.The term is not usually defined in legislation; its content is developed by case law, including European Court of Human Rights jurisprudence applied via the Human Rights Act 1998 (UK) and the ECHR Act 2003 (Ireland). In judicial review across England & Wales, Scotland, Northern Ireland and Ireland, arbitrariness commonly signals illegality for irrationality (Wednesbury unreasonableness) or lack of proportionality, taking into account irrelevant factors, ignoring relevant ones, inconsistency, or failure to give reasons.Under Article 5 ECHR, detention must not be arbitrary: it must be lawful, foreseeable, carried out in good faith, and accompanied by adequate procedural safeguards. Similar non‑arbitrariness standards govern interferences with privacy and property, policing powers (e.g. stop and search), and immigration detention.In private law, a contractual decision‑maker’s discretion must not be exercised arbitrarily; under the Braganza duty, it must be honest, rational and based on relevant considerations.Usage is broadly consistent across the UK and Ireland. Practically, arbitrary conduct risks quashing orders, declarations, release from detention, damages (e.g. false imprisonment), and costs.