em

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The letter m.
2. n. Printing The width of a square or nearly square piece of type, used as a unit of measure for matter set in that size of type.
3. n. Printing Such a measure for 12-point type; a pica.

Century Dictionary and Cyclopedia

1. n. The name of the thirteenth letter of the alphabet, usually written simply m or M.
2. n. In printing, the square of any size of type. The large square here shown is the em of the size pica; the small one , one fourth the size (one half the height and breadth), is the em of the size nonpareil, the one here used. The em is the unit of measurement in calculating the amount of type in a piece of work, as a page, a column, or a book, the standard of reckoning being 1,000; thus, this page or this book contains so many thousand, or so many thousand and hundred, ems. In the United States it is also the unit in calculating the amount of work done by a compositor, while the en is generally used for that purpose in Great Britain.
3. In colloquial speech, the objective plural of he, she, it: equivalent to them.
4. Assimilated form of en- before labials.
5. Assimilated form of en- before labials.

Wiktionary

1. n. The name of the Latin script letter [[M#|M]]/[[m#|m]].
2. n. A unit of measurement equal to the height of the type in use.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. a quad with a square body
2. n. a linear unit (1/6 inch) used in printing