When something is out-of-date, it's no longer current, accurate, or useful because newer information or better versions have replaced it. An out-of-date map might still show a highway that was torn down five years ago. An out-of-date textbook might teach scientific theories that researchers have since proven wrong.

Information becomes out-of-date at different speeds depending on the subject. A book about how to multiply fractions rarely goes out-of-date because that math hasn't changed in centuries. But a book about space exploration becomes out-of-date quickly as scientists make new discoveries and launch new missions. A phone from 2010 is very out-of-date, while a classic novel from 1850 never really goes out-of-date at all.

The phrase can also describe things that have expired, like out-of-date milk that's past its freshness date, or skills that no longer matter, like being an expert at repairing typewriters. When you update your knowledge or replace old information with new, you're making sure you're not working with out-of-date ideas. Sometimes people hyphenate the phrase (out-of-date), and sometimes they write it as three words (out of date), depending on how it's used in the sentence.