A tilde is a diacritic mark (~) put over a letter (usually a vowel) to indicate nasalization. It is also a special symbol used in mathematics, logic, and computing.
In Portuguese, ã and õ are nasalized a and o. In Spanish, tilde over n (ñ) is a separate letter (called eñe) and is a palatal [n] (SAMPA J, IPA [ɲ]), pronounced like nh in Portuguese.
The tilde was originally used as a form of contraction in Latin documents. When an n or m followed a vowel, it was often omitted, and a tilde placed over the preceding vowel.
In the International Phonetic Alphabet (IPA), the tilde is used to mark nasalization, and is placed above any phone that is nasalized.
A similar symbol, written on the line (ASCII: 126, hex 7E), is used in logic as one way of representing negation: thus ~ p means "it is not the case that p".
In Japanese, this symbol is used to indicate ranges. 12 ~ 15 means "12 to 15", ~ 3 means "up to three" and 100 ~ means "100 and greater".
In English, it is often used to mean "approximately." Therefore, ~10 would be "about 10." Similar symbols are used in mathematics, such as in π ≈ 3.14, "pi is about equal to 3.14."
Used in URLs on the World Wide Web, it often denotes a personal web page or web site which resides on the website of another company or organization. For example http://www.widgets.com/~johndoe/ might be the personal web site of John Doe, on the website of the Widgets company. This comes from the Unix shell usage of ~ followed by a username to mean the user's home directory. However, when accessed from the web, file access is usually directed to a subdirectory in the user's home directory (often called public_html or www).
See also punctuation, Õ, Special characters