To sneak means to move quietly and secretly, trying not to be noticed. When you sneak down the hallway past your sleeping baby sister's room, you're walking as softly as possible so she doesn't wake up. A cat might sneak up on a bird, keeping low and silent until the last moment.

The word often suggests you're doing something you shouldn't, or at least something you don't want others to know about. A student might sneak a peek at their phone during class, or someone might sneak a cookie before dinner. But sneaking isn't always wrong: you might sneak out of a boring assembly to use the bathroom, or sneak a gift into the house before a birthday party.

When you sneak around, you're being secretive and furtive. A sneak (as a noun) is someone who acts in underhanded or deceptive ways. A sneak attack catches someone completely off guard, like when your brother jumps out from behind a door to startle you, or in history, when one country attacks another without warning.