revolution per minute

Rotational machinery is specified in rpm because a revolution is the thing that matters: a pump moves a fixed volume per revolution, a gear tooth engages once per revolution, and a bearing sees one load cycle per revolution. Typical values anchor the scale. A car engine idles near 700 rpm and redlines between 6,000 and 8,000, a desktop hard drive spins at 5,400 or 7,200 rpm, a laboratory centrifuge reaches 15,000 rpm, and a turbocharger can exceed 150,000 rpm.

Alternating-current motors tie rpm directly to supply frequency. The synchronous speed is $N = 120f/p$, with $f$ in hertz and $p$ the number of poles, so a two-pole motor on 60 Hz turns at 3,600 rpm and a four-pole at 1,800 rpm. On 50 Hz those become 3,000 and 1,500 rpm, which is why equipment moved between North America and Europe runs 20% slower or faster unless it is redesigned or driven by a variable frequency drive. Induction motors run slightly below synchronous speed, and that gap, called slip, is what produces their torque.