The word defect has two related but distinct meanings:

1. A flaw or imperfection in something. A defect in a toy might be a wheel that won't turn properly, or a crack in the plastic. A defect in someone's vision means their eyes don't work quite right, which is why they need glasses. When factories discover defects in their products, they typically fix or recall them. The word suggests something isn't working as it should, though not every defect makes something useless. A baseball glove with a small defect in the stitching might still catch balls perfectly fine.
2. To abandon your country, group, or cause and join the opposing side. During the Cold War, some Soviet citizens defected to the West, leaving their homeland to live in America or Western Europe. A soldier who defects switches sides during a conflict. A person who does this is called a defector. This meaning carries weight because defecting can mean betraying people who trusted you, and it often involves great personal risk. It's different from simply quitting or changing your mind. People typically defect only when they believe their original side was deeply wrong or when staying has become dangerous.