·noun The portion of a line formerly occupied by the letter m, then a square type, used as a unit by which to measure the amount of printed matter on a page; the square of the body of a type.
em, i. q. eum, v. is init. ...
A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.
em interj., of wonder or emphasis, there!: em tibi, there's for you, T. ...
An Elementary Latin Dictionary