Don is to put on or wear something, like clothing or equipment. When a knight dons armor before battle, he's putting it on piece by piece. When you don your rain boots and jacket on a stormy morning, you're getting dressed for the weather. The word sounds a bit formal or old-fashioned, which is why you might see it in stories about medieval times or in descriptions of ceremonies.

The word don is actually the opposite of doff, which means to take something off. If a gentleman doffs his hat to greet someone, he's removing it politely. These days, most people just say “put on” or “wear” in everyday conversation, but don still appears in writing, especially when describing someone preparing carefully for something important. A firefighter dons protective gear before rushing into a burning building. An astronaut dons a spacesuit before a spacewalk.

You might also see don used as a title for professors at certain British universities, particularly Oxford and Cambridge. In this completely unrelated meaning, a don is a senior teacher or fellow at the university.