Definition of instrumental case in US English:

instrumental case

noun

Grammar
  • (In Sanskrit, Slavonic languages, and some other inflected languages) a case of nouns and pronouns, and of words in grammatical agreement with them, the typical function of which is to express the means or instrument with which something is done.

Origin

Early 19th century; earliest use found in William Carey (1761–1834), orientalist and missionary.