em
/em/noun
1- Printing a unit for measuring the width of printed matter, equal to the height of the type size being used【印刷】全身（西文排版行长单位, 等于所用字体的高度）。
1.1
- a unit of measurement equal to twelve points12点活字。
词源
late 18th cent.: the letter M represented as a word, since it is approximately this width.