A hi-fi is a set of electronic machines for playing recorded music as truly as possible — turntable, amplifier, speakers, and so on, built so the sound stays close to the real performance. That “true sound” idea is the fidelity in the full phrase high fidelity; hi-fi is just the short, friendly form. It can name the equipment (he bought a new hi-fi) or describe the sound (hi-fi speakers). It had a golden age from about the 1950s to the 1970s, when people were proud of a big living-room system. Today many people just stream from a phone, so hi-fi can sound a little old-fashioned, though people who care about sound still use it with a straight face. It is not the same as Wi-Fi, which only borrowed the shape of the name.