noun: a graphic symbol consisting of 2 or more letters combined (usually your initials); printed on stationery or embroidered on clothing
"Thou art Thyself the secret of Thy works;
Thou art the key: Thine image bear they all,
Or more or less. And heaven-born music, as
The balance of the force elastic, with
The gravitating force that holdeth all,—
Of all that stands on Nature's statute-book,
Image and superscription—Three in one—
In interlacing monogram doth show of Thee!"
Rev. John Andrew.
[Harmonies of Tones and Colours, Reflections on the Scheme1, page 43]