What term describes words no longer in common use, said to have originated in earlier phases of a language?

The term that describes words no longer in common use, which originated in earlier phases of a language, is archaism. Archaisms are words or expressions that have fallen out of everyday usage but may still appear in literature, historical texts, or formal contexts, reflecting the evolution of language over time. They provide a glimpse into the past and can evoke a certain tone or atmosphere in writing.

In contrast, neologism refers to the creation of new words or expressions that often arise to describe new concepts, practices, or technologies. Colloquialism pertains to informal language or slang that is characteristic of spoken language in a particular region or social group, typically recognizable in everyday conversation. Dialect involves distinct variations in language that relate to regional or social groups, encompassing unique vocabulary, pronunciation, and grammar. Archaism specifically focuses on the historical aspect of language where certain terms become obsolete, distinguishing it clearly from those other terms.