Which term describes words that are no longer used in a language?

The term that describes words that are no longer used in a language is "obsolete." This classification refers to words that have fallen out of regular usage and are typically not understood by contemporary speakers. Obsolete words might have once had specific meanings or functions within a language but have gradually been replaced or completely abandoned in favor of more modern vocabulary.

For example, terms such as "thou" or "forsooth" can be considered obsolete in contemporary English. Understanding obsolescence is crucial for examining how languages evolve over time, as it highlights the changes in society, technology, and culture that influence linguistic shifts.

Commonisation, on the other hand, refers to the process by which a once-specific term becomes a general term used widely, while semantic shift involves changes in the meanings of words over time. Broadening describes a situation where a word's meaning expands to include more than it originally did. None of these terms accurately capture the concept of words that are no longer in use, making "obsolete" the correct choice.