One-way describes something that moves or works in only a single direction, not back and forth.

A one-way street allows cars to drive in only one direction. If you turn onto a one-way street going the wrong way, you'll face oncoming traffic, which is extremely dangerous. Cities use one-way streets to keep traffic flowing smoothly in busy areas.

A one-way ticket takes you to a destination without a return trip. If you buy a one-way plane ticket from New York to Los Angeles, you'll need to buy a separate ticket to get back home. Round-trip tickets, by contrast, include both directions.

The term also describes relationships or communication that only flows one direction. A one-way conversation happens when one person talks while the other just listens without getting a chance to respond. A one-way mirror (also called a two-way mirror, confusingly) lets people on one side see through it like a window, while people on the other side see only their reflection. Police sometimes use these mirrors during witness identifications.

When something is one-way, there's no coming back the same route or reversing the process easily. That's why important decisions are sometimes called one-way doors: once you walk through them, you can't simply turn around and undo your choice.