Anti- is a prefix you add to the beginning of words to mean “against” or “opposite.” When you're anti-war, you oppose war. When something is antibacterial, it works against bacteria. When a character in a story is the antihero, they're the opposite of what we usually expect a hero to be.

You'll see this prefix everywhere once you start noticing it. Antifreeze keeps water in car engines from freezing. An antidote counteracts poison. Someone antisocial avoids spending time with others. The antithesis of an idea is its complete opposite.

Think of it as a signal that whatever follows is being opposed, prevented, or reversed. If a medicine is antiviral, it fights viruses. If a coating is anti-reflective, it prevents reflection.

Sometimes anti- creates a complete opposite (like antimatter, which is the opposite of regular matter), and sometimes it just means working against something (like anti-theft devices that prevent stealing). Either way, when you see anti- at the start of a word, you know something is being opposed or counteracted.