invalidus

Invalidus is a Latin adjective meaning weak, feeble, or infirm. It denotes lacking strength or health and is often used to describe a person, a disease, or a condition. The form is commonly given with masculine invalidus, feminine invalida, and neuter invalidum, following the standard 1st/2nd declension adjective pattern. It is derived from in- 'not' plus valere 'to be strong', contrasting with validus 'strong, healthy'.

In classical Latin, invalidus could describe someone who was ill or lacking vigor, and it could be used metaphorically for arguments or works deemed unpersuasive. The word contributed to the development of the English noun and adjective invalid, via medieval Latin and Old French, in senses relating to sickness and, later, to something not legally or logically valid.

Today, invalidus is primarily of historical or linguistic interest. It is not a modern technical term in science, but it appears in linguistic discussions of Latin morphology and in etymological notes on English words such as 'invalid' and 'invalidation'. As a Latin epithet, invalidus may occur in taxonomic names, though it has no universal or formal status outside such uses.