What term refers to old-fashioned language that is no longer in common use?

The term "antiquated language" refers specifically to words, phrases, or forms of expression that have fallen out of common usage, making them seem outdated or old-fashioned. This language may still be understood by some but is generally not used in contemporary communication. It often evokes a sense of history or nostalgia, illustrating how language evolves over time.

In contrast, the other terms provided refer to different aspects of language. Colloquial language describes informal, everyday speech that is commonly used in casual conversation but not typically found in formal writing. Modern language indicates current, widely accepted forms of expression that reflect contemporary use. Slang comprises informal words or phrases that are often unique to specific groups or communities, typically changing quickly and not always enduring over time. Thus, antiquated language is the most accurate term for expressions that are no longer in common use.