invalid

Invalid is an adjective meaning not valid, not legally binding, or not acceptable in the rules or standards governing a particular context. It can describe documents, decisions, data, or processes that fail to meet specified criteria, or that are contradicted by other rules. The term is often contrasted with valid to indicate compliance or correctness.

In logic and computing, an argument can be described as invalid if its conclusion does not necessarily follow from its premises. In software engineering or data validation, inputs, characters, or values may be labeled invalid when they fail format, type, or constraint checks. Systems typically reject or flag invalid items.

Legal and administrative usage treats something invalid as non-enforceable or void, such as an invalid contract, deed, or license. Causes include lack of consent, capacity, or illegality, as well as errors in form or verification failures. The term signals a determination that the item cannot be accepted under applicable rules.

As a noun, 'an invalid' historically referred to a person who is chronically ill or disabled. This usage is now regarded as outdated and potentially insulting; modern language prefers terms like 'a person with a disability.' The noun survives in historical writing and in some institutionalized phrases, though it is rarely appropriate in contemporary discourse.

Etymology: invalid derives from Latin invalidus meaning 'not strong' or 'unstrong', formed with in- 'not' and validus 'strong'. The sense evolved to mean not valid, and as a noun to denote a person who is bedridden or chronically ill in earlier medical usage.