Sag means to sink, droop, or hang down, usually because of weight, weakness, or time. When you hang a rope between two trees, it naturally sags in the middle, curving downward toward the ground. An old mattress might sag in the center where people have slept on it for years. Heavy snow can make tree branches sag until they nearly touch the ground.

Things can also sag when they lose their strength or firmness. A balloon sags as the air slowly leaks out. Tired shoulders sag after a long day. An old building's roof might sag from decades of supporting its own weight.

The word often describes gradual change rather than sudden collapse. A shelf doesn't break immediately under too many books; it sags bit by bit until someone notices the curve and removes some weight. Your energy level might sag in the middle of the afternoon, making you feel sluggish and tired before perking up again later. When something sags, it's drooping or bending downward, but it hasn't fallen completely.

As a noun, a sag is the downward dip itself, like the sag in a rope or the sag in an old couch cushion.