In CSS, a unit of measure based on the value of the property font-size. For example, if an element's font-size is 15px, then 1em is equal to 15px for that element alone. Another element with a font-size of 1.5cm would make 1em equivalent to 1.5cm for that element. This unit is useful for defining padding and margins that are sized relative to the text size of the element, all without having to know the text size ahead of time. For example, h1, h2, h3 {margin-top: 2em;} will give all three element types a top margin twice as big as the text size of the elements themselves.