Hz is the written short form of hertz, the unit for frequency — how many times something happens in one second. A sound of 440 Hz repeats 440 times each second. You will see Hz on headphones, speakers, screens (how many times the picture refreshes), and in science books. You write a number and then Hz, often with a space: 50 Hz. You say the full word “hertz,” not the letters H-Z. The usual written form is a capital H and a small z (Hz). The unit is named after Heinrich Hertz, a German scientist. One hertz is once per second; a kilohertz (kHz) is a thousand hertz, and a megahertz (MHz) is a million. Though actually, Hz is only for things that repeat in a regular way; it is not a word for loudness, and it is not the same as speed on the road.