Runaway describes something that's moving fast and out of control, or a person who has left home without permission.

When a horse breaks free and gallops away with no one steering it, that's a runaway horse. A truck rolling downhill with failed brakes becomes a runaway vehicle. The word captures that sense of momentum building dangerously, like when a small campfire spreads into a runaway blaze that firefighters struggle to contain.

The word also describes huge, unexpected success. A runaway bestseller sells far more copies than anyone predicted. A runaway winner in a race finishes so far ahead that second place isn't even close. Here, the “out of control” idea becomes positive: success that exceeds all expectations.

A runaway can also be a young person who leaves home without their parents' permission, usually because of serious problems. This is a difficult situation that typically requires help from trusted adults, counselors, social workers, or organizations that support young people in crisis.

In physics and engineering, runaway describes processes that accelerate themselves, like a runaway nuclear reaction where each split atom causes more atoms to split faster and faster. Scientists and engineers work hard to prevent these dangerous chain reactions.