Dictionary.net

In printing, a piece of type metal used to space between words or to fill out blank lines.

A piece of metal used to fill the void spaces between words, &c.; a mathematical instrument used in taking altitudes.

In printing, a piece of type-metal cast less in depth than the type, used to fill void spaces in a line or page, so as to leave a blank space on the paper when printed from.

n. A piece of type metal cast lower than the letters, and used to fill up the spaces between words or sentences, so as to leave a blank on the paper when printed, where it is placed.