Archaic means old-fashioned and no longer in common use, like something from an earlier time that has mostly disappeared from modern life. When you read Shakespeare and see words like “thee” and “thou” instead of “you,” those are archaic words: people understand them, but almost nobody talks that way anymore.

Things become archaic when newer alternatives replace them. Typewriters are archaic now that many people use computers. Sending telegrams is archaic now that we have phones and email. In a history museum, you might see archaic tools that people once used every day but that now seem quaint or old-fashioned.

The word often appears when discussing language. An archaic term is one that's fallen out of everyday use but still appears in old books or formal settings. For instance, “fortnight” (meaning two weeks) is archaic in American English, though British speakers still use it.

Archaic is different from obsolete: obsolete things are completely dead and gone, while archaic things still exist in special contexts.