em em /ˈɛm/

noun

1. A linear unit (1/6 inch) used in printing  ( em , pica , pica em )
   a linear unit (1/6 inch) used in printing
   
2. A quad with a square body  ( em , em quad , mutton quad )
   a quad with a square body
    Since em quad' is hard to distinguish from en quad', printers sometimes called it a mutton quad'
   since em quad' is hard to distinguish from en quad', printers sometimes called it a mutton quad'