Deadly means capable of causing death or extremely dangerous to life. A deadly poison can kill someone who swallows it. A deadly disease can end lives if not treated properly. A cobra's bite is deadly because its venom is powerful enough to kill.

The word also describes anything seriously dangerous, even if death isn't the immediate result. A deadly storm might threaten buildings and people. A deadly weapon could be anything designed to kill: a sword, a gun, or even a heavy rock used with violent intent.

In a lighter sense, people sometimes use deadly to mean extremely accurate or effective. A basketball player with a deadly three-point shot rarely misses. An archer with deadly aim hits the bullseye nearly every time.

You might hear someone describe a deadly serious situation, meaning it requires complete attention and no joking around. A classroom that's deadly quiet is so silent you could hear a pin drop. These uses borrow the word's intensity to emphasize just how extreme something is.

The related word lethal means essentially the same thing but sounds more scientific or formal. Both words remind us that some dangers in the world require real caution and respect.