A sound you make — not really a word — to tell someone to be quiet. You push air out between your teeth: "Ssh!" It is written many ways: ssh, shh, sh, shhh, with as many h's as you like. Parents say it to children, teachers say it to a noisy class, and someone in a cinema says it to the person behind them. Because it is a sound and not a normal word, you can stretch it ("Sshhhhh") to mean "be quiet NOW", or say it softly and slowly to a crying baby, where it does not mean "be quiet" at all — it means "you are safe, calm down". Adults also say it to each other in a nice way when a secret is nearly spoken: "Ssh, don't tell him." It can sound rude if a stranger says it to you, because you are being told what to do; between friends and family it is normal and gentle. There is also a verb, mostly in American English — to ssh (or shush) someone means to tell them to be quiet: "She sshed me." Note the spelling ssh is also the name of a computer program (secure shell), but that is a completely different thing, said letter by letter: S-S-H.