Tilde \Til"de\, n. [Sp., fr. L. titulus a superscription, title, token, sign. See Title, n.]

The accentual mark placed over n, and sometimes over l, in Spanish words [thus, [~n], [~l]], indicating that, in pronunciation, the sound of the following vowel is to be preceded by that of the initial, or consonantal, y.

Source: Webster's Revised Unabridged Dictionary (1913)