A concertina is a small musical instrument that looks like a hexagonal accordion. You hold it between your hands and play by squeezing and pulling it apart while pressing buttons on each end. As the instrument compresses and expands, air flows through reeds inside, creating musical notes. Unlike an accordion with piano-style keys, a concertina has small round buttons arranged in patterns.

The concertina was invented in England and Germany in the 1820s and 1830s. Sailors loved concertinas because they were compact and durable, perfect for bringing music aboard cramped ships. Folk musicians still play concertinas today, especially in Irish traditional music, where the fast, bouncy sound fits perfectly with jigs and reels.

The word also describes a type of concertina wire, which is coiled barbed wire that can stretch out and compress back together like the musical instrument. You might see this kind of security fencing at military bases or borders.

When something moves in a concertina motion, it compresses and expands repeatedly, like an accordion or a caterpillar inching along. If cars on a highway suddenly brake and bunch together, then spread out again, someone might describe it as a concertina effect.