The tilde ˜ or ~, is a grapheme with a number of uses. The name of the character came into English from Spanish, which in turn came from the Latin titulus, meaning 'title' or 'superscription'. Its primary use is as a diacritic (accent) in combination with a base letter; but, for historical reasons, it is also used in standalone form within a variety of contexts.
Text of Exeter Domesday Book of 1086
An Olivetti Lettera 32 typewriter (Portuguese Model) with tilde (and circumflex) dead-key beside Ç
Spanish typewriter (QWERTY keyboard) with dead keys for acute, circumflex, diaeresis and grave accents. Ñ/ñ is present as a precomposed character only.
Carta marina showing Finnish economy, with the captions Hic fabricantur naves and Hic fabricantur bombarde abbreviated
A diacritic is a glyph added to a letter or to a basic glyph. The term derives from the Ancient Greek διακριτικός, from διακρίνω. The word diacritic is a noun, though it is sometimes used in an attributive sense, whereas diacritical is only an adjective. Some diacritics, such as the acute ⟨á⟩, grave ⟨à⟩, and circumflex ⟨â⟩, are often called accents. Diacritics may appear above or below a letter or in some other position such as within the letter or between two letters.
Hangul, the Korean alphabet
German keyboard with umlaut letters
Blackboard used in class at Harvard shows students' efforts at placing the ü and acute accent diacritic used in Spanish orthography.