Em may refer to any of the following:
3. Em is a typeface measurement that was originally defined as a measurement between the top and bottom of the capital "M." However, today an em is a dynamic measurement based on the point size value of the font size property. For example, in CSS if you set the font size of H1 to 1.2em its text size would be 20% greater than the font size it inherits from the parent element or the default font property. See our em space definition for more information.
4. When used in chat, "'em" is chat slang for them.